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/xl-v1-bucket.go

@@ -277,7 +277,7 @@ func (xl xlObjects) DeleteBucket(ctx context.Context, bucket string) error {
 
 // SetBucketPolicy sets policy on bucket
 func (xl xlObjects) SetBucketPolicy(ctx context.Context, bucket string, policy *policy.Policy) error {
-	return savePolicyConfig(xl, bucket, policy)
+	return savePolicyConfig(ctx, xl, bucket, policy)
 }
 
 // GetBucketPolicy will get policy on bucket