Write overrides file to support app-level pipeline metadata overrides

[BenWu]

Goes with https://github.com/mozilla/mozilla-schema-generator/pull/266 to fix https://github.com/mozilla/mozilla-schema-generator/issues/264

This creates a new file called overrides that would look like:

{
  "moz_pipeline_metadata_overrides": {
    "pageload": {
      "expiration_policy": {
        "delete_after_days": 120
      }
    }
  }
}

to be used by the schema generator to apply moz_pipeline_metadata set at the app level to pings defined in a dependency.

I went with a new file instead of pings because having mostly empty ping entries might have side effects in consumers like the glean dictionary. Also pings duplicated with the dependency. It's not the cleanest solution though so I'm wondering if there's something smarter

https://github.com/mozilla/mozilla-schema-generator/pull/266 is how it's used

[BenWu]

Not needed, this can be done just in the schema generator