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/jwt_test.go

@@ -142,7 +142,7 @@ func TestWebRequestAuthenticate(t *testing.T) {
 	}
 
 	for i, testCase := range testCases {
-		gotErr := webRequestAuthenticate(testCase.req)
+		_, _, gotErr := webRequestAuthenticate(testCase.req)
 		if testCase.expectedErr != gotErr {
 			t.Errorf("Test %d, expected err %s, got %s", i+1, testCase.expectedErr, gotErr)
 		}