MediaTypeResolver has problematic hardcoded generic media type

Problem/Motivation

I'm migrating from media_acquiadam to acquia_dam.

I already ran into this issue on the problems of migrating a catch-all media type into the acquia_dam framework: #3565073: Migration to acquia_dam module fails for media type that references pdf and other files

While that new drush command is great, I was hoping to just use the "Acquia DAM: Generic" plugin for the "asset source type". This was actually recommended by Acquia support and I convinced myself tit was the easiest approach.

Except I realized this won't work. The "Acquia DAM: Generic" AssetMediaSource plugin only works if the media type is named acquia_dam_generic_asset. That because the media solver has that media type hardcoded:

    // 3. Finally fallback to the 'Generic' media type as a last resort.
    return $result ?? $this->entityTypeManager->getStorage('media_type')->load('acquia_dam_generic_asset');

It should instead hardcode the asset type and figure out what media type uses that asset type in its source plugin.

Steps to reproduce

Try to use the Acquia DAM: Generic asset type for a media type that isn't named acquia_dam_generic_asset.

The media type resolver will never resolve anything to your generic media type.

That is unless the files in the dam happen to meet the search criterion ft:{'generic_binary'}. But that's not really what we're looking for is it.

Proposed resolution

The media type resolver should instead hardcode the asset type and figure out what media type uses that asset type in its source plugin.

Remaining tasks

User interface changes

API changes

Data model changes