neon_local: use remote storage by default, add cargo neon tenant migrate (#5760)

## Problem

Currently the only way to exercise tenant migration is via python test
code. We need a convenient way for developers to do it directly in a
neon local environment.

## Summary of changes

- Add a `--num-pageservers` argument to `cargo neon init` so that it's
easy to run with multiple pageservers
- Modify default pageserver overrides in neon_local to set up `LocalFs`
remote storage, as any migration/attach/detach stuff doesn't work in the
legacy local storage mode. This also unblocks removing the pageserver's
support for the legacy local mode.
- Add a new `cargo neon tenant migrate` command that orchestrates tenant
migration, including endpoints.

test_runner/fixtures/neon_fixtures.py

@@ -1871,6 +1871,8 @@ def append_pageserver_param_overrides(
         params_to_update.append(
             f"--pageserver-config-override=remote_storage={remote_storage_toml_table}"
         )
+    else:
+        params_to_update.append('--pageserver-config-override=remote_storage=""')
 
     env_overrides = os.getenv("NEON_PAGESERVER_OVERRIDES")
     if env_overrides is not None: