tfprotov5+tfprotov6: Require EphemeralResourceServer in ProviderServer

[austinvalle]

Closes #442

This PR makes EphemeralResourceServer a required implementation for downstream provider servers, which reflects the protocol implementation.

[austinvalle]

Once we merge hashicorp/terraform-provider-corner#299, the CI errors can be resolved 👍🏻

[austinvalle]

I mistakenly added ProviderServerWithEphemeralResources when it wasn't really needed, however I double checked our downstream Go modules and the only one referencing this interface are some unit tests in terraform-plugin-mux (which should just be referencing EphemeralResourceServer) 😔

Since these are isolated to a _test package and _test.go files, they shouldn't affect the builds of providers that are upgrading terraform-plugin-go. Since we have already implemented the ephemeral resource RPCs in terraform-plugin-testing, terraform-plugin-sdk/v2 and terraform-plugin-framework, we should be good to remove this interface in the upcoming v0.26.0 release of this Go module.

Double checked this by locally updating the dns provider (muxed) and time provider (not muxed) and both pass builds with an updated terraform-plugin-go@933709cd4c2b68a7f5e5b9932f76d68b41cfe6ed and the following Go module versions:

• [email protected]
• [email protected]
• terraform-plugin-sdk/[email protected]
• [email protected]