[keyserver] Pull primaryIdentityPublicKey out of SIWE statement and pass to siweAccountCreationRequest

Summary:
Use the utility functions introduced earlier in the stack to pull primaryIdentityPublicKey out of the signed SIWE message statement (if it exists).

For native clients we expect the value of primaryIdentityPublicKey to be a string that conforms to primaryIdentityPublicKeyRegex and for web clients we expect null.

Depends on D6132

Test Plan:

1. public key appears in signing request:

2. public key properly extracted on keyserver:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6133