Remove G parameter in Global<G> and generic IdentityHandlerFactory

[udoprog]

Connections
Last step in #5105

CC @nical

Description
Removes the generic G parameter to Global<G> which is the last blocker towards starting work that needs the methods provided by it to be somewhat object safe.

A consequence of this is that in order to provide the same level of behavior in a non-generic way, we have to pass in Option<ConcreteId> for methods which supports supplying custom identifiers during construction. Long term, these methods are expected to be refactored in concert with #5121.

Testing
Ran existing test suites. This is also a breaking change, since () can no longer be used as an identifier and constructing Global doesn't require the passing in of a factory.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable.
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file. (TODO)