Remove vendored plugin server

As part of #2148, this PR removes the duplicate implementation of
`pulumigrpc.ResourceProviderServer` copied over from pulumi/pulumi and then modified.

This is a post #2258 version of #2195, and is much less invasive.

Addressing previous edits in response to
#2195 (comment):

- #1716: This change accommodated for a previous edit (`ProviderWithContext`) and is no
  longer necessary. That said, the tests it introduced are still present (and continue to
  pass), so we are sure this commit did not break the functionality.
- #1683: Simply brings DiffConfig into line with pu/pu, which we get for free by not
  vendoring.
- #1047: This is still handled in `provider_server.go`, and is still under test.
- #1065: This has already been moved out of this area in #2258, so it is no longer
  relevant here.

Author: iwahbe

pf/internal/plugin/package.go

@@ -85,9 +85,11 @@ type ProviderWithContext interface {
 	GetMappingsWithContext(ctx context.Context, key string) ([]string, error)
 
 	ParameterizeWithContext(context.Context, plugin.ParameterizeRequest) (plugin.ParameterizeResponse, error)
+
+	Attach(address string) error
 }
 
-func NewProvider(p ProviderWithContext) plugin.Provider {
+func NewProvider(p ProviderWithContext) plugin.GrpcProvider {
 	return &provider{ProviderWithContext: p}
 }
 
@@ -98,6 +100,10 @@ type provider struct {
 
 var _ plugin.Provider = (*provider)(nil)
 
+func (prov *provider) Attach(address string) error {
+	return prov.ProviderWithContext.Attach(address)
+}
+
 func (prov *provider) Pkg() tokens.Package { return prov.ProviderWithContext.Pkg() }
 
 func (prov *provider) Parameterize(