Implement Reverse Proxy Functionality with Custom Responses for FlexibleMockServer

[S-mishina]

existing state

Currently, it only functions as a single API mock server.

change state

If the communication path is as follows:

Client -> A service -> B service

In my use case, I want the A service (flexiblemockserver) to forward the request to the B service and return a constant response to the client. This allows me to test scenarios such as Istio timeouts and retry functions.

[S-mishina]

[MEMO]
In this case, the A Service should obtain the results of the B Service as they are.