Support mesh gateway mode local as a default not through proxy-defaults

#### Feature Description

In Kubernetes, we want users to be able to turn on mesh gateways via their helm configs, install two federated clusters, and have services be able to communicate between them.

Currently, that requires an additional step of creating a `proxy-defaults` config entry that sets meshGateway.mode to `local` or `remote`.

It would be great if there was a way to set this at install time so it doesn't require an extra step.

NOTE: Using the `config_entries.bootstrap` setting doesn't work anymore (that's what we used to do) because that config entry is now to be managed via CRD.

The issue names no files or tests; start by tracing the Kubernetes Helm installation settings and the existing proxy-defaults and CRD handling. Done means mesh gateway mode can be set during installation for federated clusters to communicate without separately creating a proxy-defaults entry, while avoiding the deprecated config_entries.bootstrap path.

Written by the indexing model from the issue text.