[web] Introduce getSignedIdentityKeysBlobSelector(...) to generate getSignedIdentityKeysBlob(...)

Summary:
Unlike native, where in D7035 getSignedIdentityKeysBlob(...) makes a JSI call, for web we need to construct signedIdentityKeysBlob from the Redux CryptoStore. We generate getSignedIdentityKeysBlob(...) from the new getSignedIdentityKeysBlobSelector and pass it to webGetClientResponsesSelector where it will be passed to the function generated by getClientResponsesSelector (it's a lot of creating a function within a thing that creates a function within a thing that creates a function...)

Now that we have this selector, we can conveniently reuse it in TraditionalLoginForm() and SIWELoginForm(). I'll make that refactor in a subsequent diff.

Depends on D7035

Test Plan:
See if ClientClientResponse for serverRequestTypes.SIGNED_IDENTITY_KEYS_BLOB is correctly constructed and received by keyserver:

Will also be tested in the diff where we consume getSignedIdentityKeysBlobSelector in TraditionalLoginForm() and SIWELoginForm().

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7036