Implement managed debug mode for tf5server and tf6server

[bflad]

Closes #134

This will allow providers and downstream SDK implementations to opt into a fully managed debug server, which manages reattach configuration handling and stop signals, rather than requiring duplicate logic that can be unclear and/or verbose to implement.

The tf5server and tf6server implementations could use some refactoring to reduce their duplicated code (e.g. a shared internal package), however that is left for future handling, especially with other in flight changes at the moment.

[bflad]

Verification notes (ensuring providers could be debugged successfully via VS Code):

• In terraform-provider-kubernetes, a muxed provider, the main function debug implementation simplified to only:

if *debugFlag {
	opts = append(opts, tf5server.WithManagedDebug())
}

• In terraform-plugin-framework, the new Debug flag implementation (tfsdk: Added Debug field to ServeOpts for running providers via debugger and testing processes terraform-plugin-framework#243) simplifies to the below and bflad/terraform-provider-framework debugging works just the same:

// Serve serves a provider, blocking until the context is canceled.
func Serve(ctx context.Context, providerFunc func() Provider, opts ServeOpts) error {
	var tf6serverOpts []tf6server.ServeOpt

	if opts.Debug {
		tf6serverOpts = append(tf6serverOpts, tf6server.WithManagedDebug())
	}

	return tf6server.Serve(opts.Name, func() tfprotov6.ProviderServer {
		return &server{
			p: providerFunc(),
		}
	}, tf6serverOpts...)
}

• In terraform-plugin-sdk, it would require some slight rework before being able to reduce the code, so didn't spend time there.

[ewbankkit]

LGTM 🚀.

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