[FrameworkBundle] Move AssetMapper internal artefacts in build dir - #65642

TL;DR;

AssetMapper let files publically readable (that list all assets of an app).
This fight any effort to create some "confidentiality" here
(cf all issues wanting to have admin/www assets split and free for scrutiny)

This PR moves assetmapper artefacts in build dir, but maintain per default current behaviour (with a deprecation)

Current situation

asset-map:compile currently writes two different kinds of files to the public assets
directory:

• digested assets that the web server must serve (in /public/assets per default)
• manifest.json, importmap.json and entrypoint.*.json, which are read by PHP only.

This makes internal compile metadata publicly accessible and gives it the same deployment
lifecycle as browser assets, even though the two have different consumers.

Changes

This PR adds the framework.asset_mapper.metadata_dir option. It controls only where
AssetMapper writes and reads its compiled metadata. Digested JavaScript, CSS, images and other
public assets continue to be written to the configured public assets directory.

The 8.2 default remains unchanged for backward compatibility: when the option is not set, the
metadata stays in the public assets directory. Not setting the option is deprecated when the
application actually maps assets, and the default will change to
%kernel.build_dir%/asset_mapper in 9.0.

Applications can adopt the future default immediately:

framework:
    asset_mapper:
        metadata_dir: '%kernel.build_dir%/asset_mapper'

Or they can keep the current location explicitly:

framework:
    asset_mapper:
        metadata_dir: '%kernel.project_dir%/public/assets'

Deployments that adopt the build directory must deploy it together with the public assets.

The deprecation is emitted only when AssetMapper has at least one configured application or
bundle asset path. Empty configurations do not compile metadata and therefore have no location
to choose.

--

Update: renamed compile_cache_dir in metadata_dir