Use fixed format for cache metas (#20088)

This makes cache meta files ~1.5x smaller. (I hoped together with
previous diff it will give us 4x, but it is more like 3x).
Implementation is mostly straightforward, here are some comments:
* I make `CacheMeta` a regular class, it doesn't need to be immutable
IMO (since we actually mutate it in few places).
* I remove all uses of untyped dicts in favour of `CacheMeta` (note this
might make JSON format slightly slower actually, but difference is below
noise level)
* Instead of manually checking some individual keys, I use blanket
`try/except (KeyError, ValueError)` when deserializing metas.
* In one place (where we update meta file after _read_), I update the
loaded view to match the updated file 1:1. This should be more robust.
* I still use JSON dumps for options and plugins snapshots. Serializing
these using FF is tricky (and will be simpler with type tags).
* I rename `data_json`/`meta_json` paths to `data_file`/`meta_file`
everywhere.

Author: ilevkivskyi

mypy-requirements.txt

@@ -4,4 +4,4 @@ typing_extensions>=4.6.0
 mypy_extensions>=1.0.0
 pathspec>=0.9.0
 tomli>=1.1.0; python_version<'3.11'
-librt>=0.2.1
+librt>=0.3.0