Create app-level metadata for communication between the build system and debugger

This issue tracks work related to generation and use of app-level metadata in several repos (to be created and added here later)

- SDK:
- define app-level metadata format (documentation and types) https://github.com/dart-lang/sdk/issues/45477
- builds: generate app metadata, merged app-level metadata (for multi-ddc scenario)
- generate metadata for SDK libraries
- build_web_compilers:
- blaze
- flutter tools
- dwds: use app metadata
- read app level metadata to
- find locations of individual metadata files
- set app-level properties, such as root lib, null safety, sdk module and libraries, and more
- builds: remove generating merged metadata
- build_web_compilers:
- blaze
- flutter tools

Needed for:
https://github.com/dart-lang/webdev/issues/1226
https://github.com/dart-lang/sdk/issues/44760
https://github.com/dart-lang/webdev/issues/1030
Better implementation of: https://github.com/dart-lang/webdev/issues/1457 (see discussion in https://github.com/dart-lang/webdev/pull/1472)

Start with the SDK metadata format and types in issue 45477, then trace the build_web_compilers, blaze, Flutter tools, and dwds work described here. Done means app-level metadata is generated and consumed across the listed build and debugger components, including multi-DDC cases, while obsolete merged-metadata generation is removed.

Written by the indexing model from the issue text.