Support environment variable replacements in manifest config

[atzoum]

We are currently using the release-please-action v3 where we can set dynamic release-please configuration options using environment variables in the action's configuration.
However, we cannot switch to release please v4 since there is no way for us to achieve the same dynamic configuration through a static release-please-config.json file.

It would be great if we could use any kind of templating mechanism in the json file itself so that we could introduce the dynamic behaviour we want to, e.g.

{
  "release-as": "${{ env.SOME_VALUE || 'default' }}"
}

This could unlock for us a lot of new usage patterns of release please in our repositories :)