tfprotov5+tfprotov6: Update to protocol versions 5.4/6.4 with GetMetadata RPC and GetProviderSchemaOptional server capability

[bflad]

Closes #310

The CI failures for terraform-provider-corner are expected in this case. Protocol upgrades that impose new RPCs will either require:

• The tfprotov5.ProviderServer/tfprotov6.ProviderServer interface to require a new method
• Or, new "optional" interfaces be implemented (let's make up ProviderServerWithGetMetadata in this case)

terraform-plugin-go is a low level abstraction which is designed to directly implement the protocol rather than introduce its own abstractions. Most provider developers will directly interface with higher level Go modules, such as terraform-plugin-sdk and terraform-plugin-framework. Except for advanced provider development using this Go module directly, this type of low level "breaking" change will be hidden by also upgrading those Go modules at the same time:

• helper/schema: Update to protocol version 5.4 and enable GetProviderSchemaOptional server capability terraform-plugin-sdk#1235
• providerserver: Update to protocol versions 5.4/6.4 and enable GetProviderSchemaOptional server capability terraform-plugin-framework#829
• all: Update to protocol version 5.4 and 6.4 terraform-plugin-mux#186

Therefore the change is implemented on the existing interface. terraform-provider-corner happens to also implement its own terraform-plugin-go based provider server code, which is affected by this change. Upgrading the various dependencies and implementing the new method makes terraform-provider-corner testing successful, which can be seen with hashicorp/terraform-provider-corner#174. The CI of this Go module and terraform-provider-corner implementation details just happens to be a chicken-and-egg problem which tends to only come up in this particular type of functionality change. 🐔 🥚

[austinvalle]

LGTM 🚀

[bendbennett]

LGTM 🚀

[bflad]

GetProviderSchemaOptional works as expected with upstream hashicorp/terraform#33807 and downstream hashicorp/terraform-plugin-framework#829 / hashicorp/terraform-plugin-sdk#1235 / hashicorp/terraform-plugin-mux#186. Merging!

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