[keyserver] Store SIWESocialProof in social_proof column of cookies table

Summary:
Stringify the SIWESocialProof object and store in the social_proof column of the cookies table.

Depends on D6135

Test Plan:
public_key and social_proof columns populated as expected:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6136