feat: update instance workflow

[kaustavb12]

Description

The PR adds an "Update Instance" workflow takes an input the name of an existing instance and the config in JSON format and merged the provided config to the intances's existing config.yml file.

Testing instructions

1. Update an existing cluster to add the update-instance.yml under the cluster's .github/workflows path.
2. Trigger the update instance workflow with a valid intance name and a valid json formatted config. Remember to escape the double quotes in the json string, Ex. {\"EDX_PLATFORM_VERSION\": \"release/teak\"}

Other information

Private Ref : BB-9946

[gabor-boros]

I just checked the code and already executed workflow. Feel free to go ahead and merge it!