mcp,design: revert 'content' back to an interface type (#29)

After some experience with the flattened version of Content, we see that
it can easily lead to incorrect usage and is a harder API to read.
Therefore, this CL changes back to a design similar to what we had prior
to https://go.dev/cl/672415, though opting to promote content
unmarshalling to the protocol types that use it, which leads to an
overall simpler api.

mcp/server.go

@@ -412,7 +412,9 @@ func fileResourceHandler(dir string) ResourceHandler {
 			return nil, err
 		}
 		// TODO(jba): figure out mime type. Omit for now: Server.readResource will fill it in.
-		return &ReadResourceResult{Contents: []*ResourceContents{NewBlobResourceContents(params.URI, "", data)}}, nil
+		return &ReadResourceResult{Contents: []*ResourceContents{
+			&ResourceContents{URI: params.URI, Blob: data},
+		}}, nil
 	}
 }