We move the code responsible for the instantiation of
`LoggerContextFactory` and `ThreadContextMap` from the static entry
points to the logging system (`LogManager` and `ThreadContext`) to the
`Provider` class.

The `Provider` class is instantiated using `ServiceLoader`, so
`log4j-core` 2.x and 3.x can reimplement the initialization process
according to their own rules. E.g. `log4j-core` 3.x can use the DI to
create an instance of `LoggerContextFactory` and `ThreadContextMap`.

The following modification were performed:

* a **new** system property `log4j.provider` was introduced,
* the old `log4j2.loggerContextFactory` has been deprecated and revised: if set it
  selects the first provider that uses the given `LoggerContextFactory`.
  Therefore it selects now both the context factory and thread context
  map implementations,
* private static configuration values were removed from
  `ThreadContextMap` implementations, helping test parallelisation,
* a distinct `NoOpThreadContextStack` implementation has been
  introduced.

We make `ServiceLoaderUtil` more GraalVM-friendly by replacing the usage
of `MethodHandles.Lookup` with the requirement for the caller to
instantiate the `ServiceLoader` himself.

Bumps [codecov/codecov-action](https://github.com/codecov/codecov-action) from 4.0.2 to 4.3.0.
- [Release notes](https://github.com/codecov/codecov-action/releases)
- [Changelog](https://github.com/codecov/codecov-action/blob/main/CHANGELOG.md)
- [Commits](codecov/codecov-action@0cfda1d...8450866)

---
updated-dependencies:
- dependency-name: codecov/codecov-action
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>