helper/schema: Add helper function JsonEnvDefaultFunc

[gw0]

Currently, there are only two simple helper functions available for providing default values in Terraform plugin schema definitions. These read env variables and can only return a single string.

This PR adds a helper function JsonEnvDefaultFunc that can be used in more complex scenarios that require a map with arbitrary keys (to be used with TypeMap). It expects the map to be encoded as an JSON object in a string.

For example if you have a provider that needs support for arbitrary HTTP headers:

provider "foo" {
  url  = "http://..."
  headers = {
    "Cookies" = "foo=bar"
  }
}

This helper function opens the door for passing HTTP headers using an env variable:

$ FOO_HTTP_HEADERS='{"Cookies":"foo=bar"}' terraform plan

[gw0]

Rebased and corrected gofmt check.

[hashicorp-cla]

All committers have signed the CLA.