Add request signing accepting different methods for request signature

[iamKunalGupta]

Assuming the following metadata-keys/headers are available on request:

• x-peerdb-request-signature: v1,<Base64EncodedSignature>
• x-peerdb-request-type: digest/raw
• x-peerdb-request-timestamp: (optional for now), the time when request was created, will have tolerance check to prevent replay attacks
• x-peerdb-request-signing-key-id: keyID for signing
• x-peerdb-request-signing-alg: algorithm used for signing

The following env vars are used:

• PEERDB_REQUEST_SIGNING_VERIFICATION_ENABLED: true/false
• PEERDB_REQUEST_SIGNING_KEY_TYPE: AWS_KMS_SIGN for now
• (optional) PEERDB_REQUEST_SIGNING_TIMESTAMP_TOLERANCE_SECONDS
• PEERDB_REQUEST_SIGNING_KEYS: json map of allowed key IDs, map of keyID to keyValue, in case of KMS, keyId and keyValue will be same, will allow for key rotation as well as using different keys for different entities.

Request will only be accepted if:

• x-peerdb-request-signing-key-id is present in PEERDB_REQUEST_SIGNING_KEYS
• Concat timestamp + message, and verify with the remaining parameters
• Optionally check for timestamp tolerance.

Current plan is to only have this on certain GRPC methods and implement for AWS Asymmetric keys. This will NOT work on HTTP requests, a separate interceptor can be added for HTTP request body with common methods.

[iamKunalGupta]

Closing since GRPC raw body intercept is a pain and if we plan to Marshal back to json, the result may not be reproducible as to what the client sent