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.