[core] Add ROOT_LOG env var support for RLogger

[deveshbervar]

Why

Enables per-channel verbosity to be configured at startup via an
environment variable, without recompiling ROOT. This allows tracing
(e.g. CMS autoparsing after #7609) by simply setting ROOT_LOG.

What

Setting ROOT_LOG at startup now configures RLogger channel verbosity:

export ROOT_LOG='ROOT.InterpreterPerf=Debug(3),ROOT.RBrowser=Error'

Supported levels: Fatal, Error, Warning, Info, Debug, Debug(N).
gDebug is also applied as a global verbosity floor.

Changes

• RLogger.hxx: added fEnvVerbosity map to RLogManager, added
  GetEnvVerbosity(), updated GetEffectiveVerbosity() to check
  per-channel env overrides before falling back to global verbosity
• RLogger.cxx: constructor now parses ROOT_LOG and applies gDebug

Tests

Since RLogManager is a static singleton initialized once at process
startup, setenv() mid-test does not affect it. I would appreciate
guidance from the maintainer on the recommended approach for testing
this feature. Happy to add tests based on your feedback.

[deveshbervar]

@dpiparo ping! PR ready for review. I have implemented the ROOT_LOG
environment variable support as described in the issue. Please let me
know if any changes are needed, especially regarding the test approach
for the singleton RLogManager. Thank you!

[dpiparo]

Is testing perhaps still missing?

[deveshbervar]

Hi @dpiparo, tests have been added in core/foundation/test/RLoggerEnvVar.cxx.

Since RLogManager is a static singleton that parses ROOT_LOG once at
process startup, I used CMake's set_tests_properties ENVIRONMENT to
set ROOT_LOG before the test process starts, rather than setenv() mid-test.

Please let me know if this approach is acceptable or if you'd prefer
a different testing strategy. Thank you!

[pcanal]

Why are those code comments no longer relevant/helpful?

[deveshbervar]

Hi @pcanal,
Sorry, those comments were accidentally removed during editing.
Restored them now.