tfprotov5/tf5server+tfprotov6/tf6server: Add downstream RPC request duration and response diagnostics logging

[bflad]

Closes #183

[bflad]

This is now verified via unit testing and I also ensured it worked with a real provider:

2022-07-01T08:36:45.801-0400 [TRACE] sdk.proto: Received downstream response: tf_proto_version=6.0 tf_provider_addr=registry.terraform.io/hashicorp/tls tf_req_id=074ddc8e-8c18-13bc-269b-b58fcb30a223 tf_resource_type=tls_self_signed_cert tf_rpc=ApplyResourceChange diagnostic_error_count=1 diagnostic_warning_count=0 tf_req_duration_ms=1
2022-07-01T08:36:45.801-0400 [ERROR] sdk.proto: Response contains error diagnostic: tf_proto_version=6.0 tf_provider_addr=registry.terraform.io/hashicorp/tls tf_req_id=074ddc8e-8c18-13bc-269b-b58fcb30a223 tf_resource_type=tls_self_signed_cert tf_rpc=ApplyResourceChange diagnostic_detail="Could not determine Authority Key Identifier" diagnostic_severity=ERROR diagnostic_summary="Invalid Certificate Authority"
2022-07-01T08:36:45.801-0400 [TRACE] sdk.proto: Served request: tf_proto_version=6.0 tf_provider_addr=registry.terraform.io/hashicorp/tls tf_req_id=074ddc8e-8c18-13bc-269b-b58fcb30a223 tf_resource_type=tls_self_signed_cert tf_rpc=ApplyResourceChange
2022-07-01T08:36:45.866-0400 [TRACE] sdk.helper_resource: Called Terraform CLI apply command: test_name=TestResourceSelfSignedCert_InvalidConfigs test_step_number=5 test_terraform_path=/usr/local/bin/terraform test_working_directory=/var/folders/w8/05f3x02n27x72g0mc2jy6_180000gp/T/plugintest3614426453
2022-07-01T08:36:45.866-0400 [WARN]  sdk.helper_resource: Error running Terraform CLI command: test_name=TestResourceSelfSignedCert_InvalidConfigs test_step_number=5 test_terraform_path=/usr/local/bin/terraform test_working_directory=/var/folders/w8/05f3x02n27x72g0mc2jy6_180000gp/T/plugintest3614426453
  error=
  | exit status 1
  |
  | Error: Invalid Certificate Authority
  |
  |   with tls_self_signed_cert.test,
  |   on terraform_plugin_test.tf line 2, in resource "tls_self_signed_cert" "test":
  |    2: 					resource "tls_self_signed_cert" "test" {
  |
  | Could not determine Authority Key Identifier

[bendbennett]

LGTM

[detro]

LGTM, but I do have a question in regards to the use of struct instead of string for storing the requestStart time in the context.

I'm sure I'm missing something.