[keyserver] Handle SIGNED_IDENTITY_KEYS_BLOB response

Summary:
When we receive a SIGNED_IDENTITY_KEYS_BLOB response from the client, we want to verify the signature of the payload and then update the signed_identity_keys column of the cookies row in the DB.

(I didn't know what the right approach for error-handling here was, so I'm just continueing through the loop if the signedIdentityKeysBlob object is malformed?)

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Depends on D7070

Test Plan:

1. Nuke signed_identity_keys column for all rows of cookies table:

2. Refresh the page on web so handleInitialClientSocketMessage gets hit on the keyserver
3. Observe that the web client constructs and sends a valid signedIdentityKeysBlob by setting breakpoints in keyserver:

4. Step through processClientResponses on the keyserver and make sure cookies table is updated as expected:

5. "Refresh" the native app running in iOS Simulator so handleInitialClientSocketMessage gets hit on the keyserver.
6. Observe that the native client constructs and sends a valid signedIdentityKeysBlob by setting breakpoints in keyserver:

7. Step through processClientResponses on the keyserver and make sure cookies table is updated as expected:

Reviewers: ashoat, marcin

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7082