required changes for handle.invalid spec change

[bnewbold]

We've got work in progress in the typescript implementation to transmit handle.invalid for known repos when the handle fails to cross-verify. Some places indigo might need to do something special about this, with a focus on stuff in active development (not, eg the golang PDS impl):

• search (palomar) should not index the value handle.invalid
• XRPC server endpoints should disallow handle.invalid as a query parameter for params with string format handle. this could come for free if we just check at validation or db insert time, instead of every API query param?
• BGS should return this value if handle verification fails. not sure if this is high priority

(possibly more, these were just the first I could think of)