Include `signedIdentityKeysBlob` in `siwe_auth` request and update `siweAuthRequestInputValidator` to accept `signedIdentityKeysBlob`. Right now we don't do anything with the `signedIdentityKeysBlob`, but it's being passed to `keyserver` from `web` when hitting both `login` and `siwe_auth` endpoints.
Next up is
1. Making sure that `signedIdentityKeysBlob` is valid (regex checks)
2. Pulling out `primary.ed25519` and verifying the `payload` signature
(maybe an argument for leaving `primaryIdentityPublicKey` as part of the `login` request and maybe adding it to `siwe_auth`?)
3. Stringifying the entire `signedIdentityKeysBlob` and storing it in the serverDB (going to do a migration to remove existing stuff first).
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Depends on D6895