[Phase 1] Multiplex provider - minimal TF Plugin Framework provider implementation

[fairclothjm]

Description

We introduces a minimal first step for combining (using terraform-plugin-mux) into a single binary the current resource and data sources implemented using Terraform Plugin SDK v2 with other future implementations (such as using Terraform Plugin Framework).

This PR

Phase 1

• Introduces (in internal/provider/fwprovider) a minimal provider implementation that uses the TF Plugin Framework and muxes it with the current SDK v2-based provider's server. This new skeleton provider
• Implements 0 resources and data sources
• Is required to declare a provider schema that is identical to the schema(s) declared by all other providers in the muxed provider server
• Removes Default values from the SDK v2-based provider's schema declaration as the TF Framework-based provider does not have that facility. Defaults should be handled in the provider's ConfigureFunc

Previous Phases

Phase 0

• https://github.com/hashicorp/terraform-provider-vault/pull/2059
• Adds a multiplexer that combines the current Terraform Plugin SDK v2 Provider implementation with 0 additional providers
• Enables support for both SDKv2 and TF Plugin Framework

Subsequent Phases

Phase 2

• Completes the provider configuration setup
• Complete all auth login setup

Phase 3

• Migrate a resource/data source to the TF Plugin Framework