Lay the groundwork for supporting IP identifiers

[aarongable]

Clean up how we handle identifiers throughout the Boulder codebase by

• moving the Identifier protobuf message definition from sa.proto to core.proto;
• adding support for IP identifier to the "identifier" package;
• renaming the "identifier" package's exported names to be clearer; and
• ensuring we use the identifier package's helper functions everywhere we can.

This will make future work to actually respect identifier types (such as in Authorization and Order protobuf messages) simpler and easier to review.

Part of #7311