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/routers.go

@@ -99,6 +99,9 @@ func configureServerHandler(endpoints EndpointList) (http.Handler, error) {
 		registerDistXLRouters(router, endpoints)
 	}
 
+	// Add STS router only enabled if etcd is configured.
+	registerSTSRouter(router)
+
 	// Add Admin RPC router
 	registerAdminRPCRouter(router)