[lib/keyserver] make localID optional in the sendReactionMessageRequestInputValidator

Summary:
make localID optional in the sendReactionMessageRequestInputValidator. We need to do this because old native clients ie. build 172 does not have a localID field and by previously making localID requried it would cause the sendReactionMessageRequestInputValidator to fail

Depends on D6244

Test Plan:
Tested both older and newer native clients.

To test newer clients, I sent a reaction message and everything worked as expected, I checked the DB and the reaction message had the creation column filled.

To test older clients, I checked out the commit with the release tag using git checkout v1.0.172. This is to test an older native client. Then I copied and pasted the necessary code in the message-responders file to demonstrate an updated keyserver, and when I tried sending a reaction message, everything worked as expected and when I checked the DB, the reaction message did not have the creation column filled which makes sense since no localID was passed

Reviewers: atul, ashoat, tomek

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6245