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

pkg/madmin/examples/set-credentials.go

@@ -36,7 +36,7 @@ func main() {
 		log.Fatalln(err)
 	}
 
-	err = madmClnt.SetCredentials("YOUR-NEW-ACCESSKEY", "YOUR-NEW-SECRETKEY")
+	err = madmClnt.SetAdminCredentials("YOUR-NEW-ACCESSKEY", "YOUR-NEW-SECRETKEY")
 	if err != nil {
 		log.Fatalln(err)
 	}