Support custom annotation to assert that a function is an initializer

[ericglau]

When validating errors in parent initializer calls, initializers are detected based on the function name or modifier.

In cases where the user's initializer is named differently or has differently named modifiers, allow users to annotate that a function is an initializer. See https://github.com/OpenZeppelin/openzeppelin-upgrades/pull/1095#discussion_r1890445928