tfprotov5+tfprotov6: Add EphemeralResourceServer to ProviderServer interface

[austinvalle]

terraform-plugin-go version

v0.25.0 (not yet released)

Proposal

Following #441, the tfprotov5 and tfprotov6 packages will contain a new EphemeralResourceServer interface which contains the RPC handlers for ephemeral resource type support. This is initially introduced as a temporary interface to avoid confusing Go build errors when upgrading just terraform-plugin-go, before the RPC changes are implemented upstream in terraform-plugin-mux, terraform-plugin-framework and terraform-plugin-sdk/v2.

We should remove the temporary interface ServerWithEphemeralResources and type assertions once all downstream SDKs are updated and released, and add EphemeralResourceServer to the main ProviderServer interface