Goal: 

Currently in llama stack there is no way to modify a provider's configuration after standing up the server. This was described in an RFC I wrote: https://github.com/meta-llama/llama-stack/pull/1359

This is an issue for fields in the training provider configuration like `model_path`, `data_path` etc which are runtime arguments but must be specified at server creation.

Add a provider modification API to the `/providers` API that can be used to patch existing provider APIs. One should not be able to add new providers using this route, only modify existing providers the stack has.

Acceptance Criteria:

there is a `/providers` API for modifying provider configuration that has proper testing and can be used with the InstructLab KFT provider.