Changes proposed

Add support for PATCH as a save_method, currently only PUT is supported as a save_method

• use JSONPatchAction to validate and read JSON patch operations
• add patch_policy_data to opal_client which does a PATCH API call to OPA
  • based on the save_method value, call either set_policy_data or patch_policy_data respectively
• added jsonpatch package to apply JSON patch operations on OPA data cache when offline mode is enabled
• added a function custom_encoder for using it as the default encoder for doing json.dumps on the Union type JsonableValue to ignore default fields values and dump based on alias name

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• This PR does not contain plagiarized content.
• The title of my pull request is a short description of the requested changes.

Note to reviewers

Need thoughts/help on if there is a better way to do custom_encoder implementation, haven't used pydantic before

the reason i used it was because FastAPI does by default print response using JSON alias names but we are not returning response from FastAPI server but rather feeding the model directly to OPA in which case the field name would be used when doing a json.dumps instead of alias name

and also to remove default None value populated fields from the dumped JSON for any request that matches the JSONPatchAction type

One more thing is, OPA does not support move operation as a JSON patch operation, refer discussion and hence OPAL would also not support it
what would be a good place in the docs to mentions this as a NOTE ?