Introduce STS client grants API and OPA policy integration (#6168)

This PR introduces two new features

- AWS STS compatible STS API named AssumeRoleWithClientGrants

```
POST /?Action=AssumeRoleWithClientGrants&Token=<jwt>
```

This API endpoint returns temporary access credentials, access
tokens signature types supported by this API

  - RSA keys
  - ECDSA keys

Fetches the required public key from the JWKS endpoints, provides
them as rsa or ecdsa public keys.

- External policy engine support, in this case OPA policy engine

- Credentials are stored on disks

cmd/object-api-errors.go

@@ -378,12 +378,6 @@ func (e BackendDown) Error() string {
 	return "Backend down"
 }
 
-// isErrIncompleteBody - Check if error type is IncompleteBody.
-func isErrIncompleteBody(err error) bool {
-	_, ok := err.(IncompleteBody)
-	return ok
-}
-
 // isErrObjectNotFound - Check if error type is ObjectNotFound.
 func isErrObjectNotFound(err error) bool {
 	_, ok := err.(ObjectNotFound)