Split request_signature into separate paths that return JSON

[jclapis]

This PR does a few things:

1. Splits the old /request_signature route of the signer service, which took a type, into three subroutes with the types baked in:

• /request_signature/bls
• /request_signature/proxy-bls
• /request_signature/proxy-ecdsa

2. Refactors the response from the above to return a full JSON object rather than just a raw signature string, e.g.:

{
  "pubkey": "0x883827193f7627cd04e621e1e8d56498362a52b2a30c9a1c72036eb935c4278dee23d38a24d2f7dda62689886f0c39f4",
  "object_root": "0x0123456789012345678901234567890123456789012345678901234567890123",
  "module_signing_id": "0x6a33a23ef26a4836979edff86c493a69b26ccf0b4a16491a815a13787657431b",
  "signature": "0xa43e623f009e615faa3987368f64d6286a4103de70e9a81d82562c50c91eae2d5d6fb9db9fe943aa8ee42fd92d8210c1149f25ed6aa72a557d74a0ed5646fdd0e8255ec58e3e2931695fe913863ba0cdf90d29f651bce0a34169a6f6ce5b3115"
}

This provides a complete capture of everything needed to verify a signature so it can be routed into middleware without needing prior any context for validation.