Add Context.Key.create and createWithDefault

We'd like to remove needless differences between Attributes.Key, CallOptions.Key, and Context.Key. The Context.key() and keyWithDefault() is actually a bit weird today, and not something we'd like to copy. But we'd like to add the create methods to Key for consistency.

We will leave the current key() and keyWithDefault() methods. Just add new methods.

Related: #1869, #1764.

Start by comparing Context.Key with Attributes.Key and CallOptions.Key, focusing on the existing Context.key() and keyWithDefault() methods and the related issues #1869 and #1764. Confirm the new API should add create and createWithDefault while preserving the current methods; done means the Context.Key API is consistent with the other key types.

Vom Indexierungsmodell aus dem Issue-Text verfasst.