feat: add ES256 signing keys backed by AWS KMS

[hf]

Adds initial support for JWT signing keys backed by AWS KMS. Only ES256 support is added, as these are $0.15 per 10k requests of the Sign API while RSA is extremely expensive.

You can specify it in the GOTRUE_JWT_KEYS JSON, like so:

{
  "kty": "EC",
  "aws:kms:arn": "arn:aws:kms:us-east-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab",
  "use": "sig",
  // ...
}

It will use the default AWS credentials assigned to the process / machine.