Ensures namespace enums are marked as Sendable

[rafaelcepeda]

Changed code generation to ensure enum used as namespace are always marked as Sendable

Motivation:

Code that relies on grpc-swift-2 code generation capability cannot build with -require-explicit-sendable compiler flag.

Modifications:

Modified places where EnumDescription is built for namespace to include conformance with Sendable.

Result:

Generated code now works with -require-explicit-sendable compiler flag enabled.

[glbrntt]

Thanks @rafaelcepeda!