Implement merge_prop_transformer configuration

[pedrofs]

I'm using the prop_transformer to maintain a clear naming convention between React and Rails: React is camelCase and Rails is snake_case.

Unfortunately, I faced an inconsistency when trying to work with the InertiaRails.merge functionality. See #266 .

This PR introduces the merge_prop_transformer configuration, similar to the prop_transformer. It'll receive the array of merge keys, and you can transform them as you wish.

[bknoles]

We've talked about this and decided that the JS side is the better place to do key transformation. That way we avoid having to worry about transforming the keys that come back to the server from the React app @skryukov built an NPM packages that handles converting to camelCase for Inertia apps that does this all very cleanly:

https://github.com/skryukov/inertia-caseshift

We'd recommend using that instead of transforming props in Rails.

We're going to deprecate the Inertia Rails prop transformation feature (instead of improving it) and eventually remove it.

That said, thanks for submitting the PR and sorry for this sitting around for so long!