Add reserved IP related resources and data source

[andrewsomething]

This PR renames the existing floating IP resources and data source to use the new, upcoming reserved IP branding.

$ TF_ACC=1 go test -v --run 'Reserved|Floating' ./...
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDataSourceDigitalOceanFloatingIp_Basic
=== PAUSE TestAccDataSourceDigitalOceanFloatingIp_Basic
=== RUN   TestAccDataSourceDigitalOceanReservedIP_Basic
=== PAUSE TestAccDataSourceDigitalOceanReservedIP_Basic
=== RUN   TestAccDataSourceDigitalOceanReservedIP_FindsFloatingIP
=== PAUSE TestAccDataSourceDigitalOceanReservedIP_FindsFloatingIP
=== RUN   TestAccDigitalOceanFloatingIPAssignment_importBasic
=== PAUSE TestAccDigitalOceanFloatingIPAssignment_importBasic
=== RUN   TestAccDigitalOceanFloatingIP_importBasicRegion
=== PAUSE TestAccDigitalOceanFloatingIP_importBasicRegion
=== RUN   TestAccDigitalOceanFloatingIP_importBasicDroplet
=== PAUSE TestAccDigitalOceanFloatingIP_importBasicDroplet
=== RUN   TestAccDigitalOceanReservedIP_importBasicRegion
=== PAUSE TestAccDigitalOceanReservedIP_importBasicRegion
=== RUN   TestAccDigitalOceanReservedIP_importBasicDroplet
=== PAUSE TestAccDigitalOceanReservedIP_importBasicDroplet
=== RUN   TestAccDigitalOceanFloatingIPAssignment
=== PAUSE TestAccDigitalOceanFloatingIPAssignment
=== RUN   TestAccDigitalOceanFloatingIPAssignment_createBeforeDestroy
=== PAUSE TestAccDigitalOceanFloatingIPAssignment_createBeforeDestroy
=== RUN   TestAccDigitalOceanFloatingIP_Region
=== PAUSE TestAccDigitalOceanFloatingIP_Region
=== RUN   TestAccDigitalOceanFloatingIP_Droplet
=== PAUSE TestAccDigitalOceanFloatingIP_Droplet
=== RUN   TestAccDigitalOceanReservedIPAssignment
=== PAUSE TestAccDigitalOceanReservedIPAssignment
=== RUN   TestAccDigitalOceanReservedIPAssignment_createBeforeDestroy
=== PAUSE TestAccDigitalOceanReservedIPAssignment_createBeforeDestroy
=== RUN   TestAccDigitalOceanReservedIP_Region
=== PAUSE TestAccDigitalOceanReservedIP_Region
=== RUN   TestAccDigitalOceanReservedIP_Droplet
=== PAUSE TestAccDigitalOceanReservedIP_Droplet
=== CONT  TestAccDataSourceDigitalOceanFloatingIp_Basic
=== CONT  TestAccDigitalOceanReservedIP_Droplet
=== CONT  TestAccDigitalOceanReservedIP_Region
=== CONT  TestAccDigitalOceanReservedIPAssignment_createBeforeDestroy
=== CONT  TestAccDigitalOceanReservedIPAssignment
=== CONT  TestAccDigitalOceanFloatingIP_Droplet
=== CONT  TestAccDigitalOceanReservedIP_importBasicRegion
=== CONT  TestAccDigitalOceanFloatingIP_Region
--- PASS: TestAccDigitalOceanReservedIP_Region (5.93s)
=== CONT  TestAccDigitalOceanFloatingIP_importBasicDroplet
--- PASS: TestAccDigitalOceanFloatingIP_Region (7.72s)
=== CONT  TestAccDigitalOceanFloatingIP_importBasicRegion
--- PASS: TestAccDigitalOceanReservedIP_importBasicRegion (9.20s)
=== CONT  TestAccDigitalOceanFloatingIPAssignment_importBasic
--- PASS: TestAccDataSourceDigitalOceanFloatingIp_Basic (9.28s)
=== CONT  TestAccDataSourceDigitalOceanReservedIP_FindsFloatingIP
--- PASS: TestAccDigitalOceanFloatingIP_importBasicRegion (7.69s)
=== CONT  TestAccDataSourceDigitalOceanReservedIP_Basic
--- PASS: TestAccDataSourceDigitalOceanReservedIP_FindsFloatingIP (7.12s)
=== CONT  TestAccDigitalOceanFloatingIPAssignment_createBeforeDestroy
--- PASS: TestAccDataSourceDigitalOceanReservedIP_Basic (13.84s)
=== CONT  TestAccDigitalOceanFloatingIPAssignment
--- PASS: TestAccDigitalOceanFloatingIP_importBasicDroplet (82.37s)
=== CONT  TestAccDigitalOceanReservedIP_importBasicDroplet
--- PASS: TestAccDigitalOceanFloatingIPAssignment_importBasic (103.50s)
--- PASS: TestAccDigitalOceanReservedIPAssignment (133.51s)
--- PASS: TestAccDigitalOceanFloatingIP_Droplet (145.38s)
--- PASS: TestAccDigitalOceanReservedIP_Droplet (150.73s)
--- PASS: TestAccDigitalOceanFloatingIPAssignment (137.28s)
--- PASS: TestAccDigitalOceanReservedIP_importBasicDroplet (94.00s)
--- PASS: TestAccDigitalOceanFloatingIPAssignment_createBeforeDestroy (190.16s)
--- PASS: TestAccDigitalOceanReservedIPAssignment_createBeforeDestroy (319.98s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean    320.018s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/mutexkv        (cached) [no tests to run]

[bentranter]

Three potential spots where the docs are a bit confusing, other than that LGTM

[scotchneat]

LGTM