MSCommon: add custom formatter for debug log records

[jcbrill]

Add a custom formatter to the MSCommon/common.py debug logging that accepts an optional class name.

Changes:

• Add support for optional extra dictionary for the python logging.
• Allow passing 'classname' via the extra dictionary and creating the log records with '%(classname)s.%(funcName)s' for class methods.
• Add a method to populate and return the extra dictionary.
• Add a boolean constant indicating if debugging is enabled or not.

Example code demonstrating functionality:

from SCons.Tool.MSCommon.common import (
    debug,
    debug_extra,
    DEBUG_ENABLED,
)

class DebugExample:

    debug_extra = None
    
    info = {
        '1': 'One',
        '2': 'Two',
        '3': 'Three',
    }

    @classmethod
    def setup(cls):
        cls.debug_extra = debug_extra(cls)

    @classmethod
    def debug_dump(cls):
        for key, val in cls.info.items():
            debug('info: key=%s, val=%s', key, val, extra=cls.debug_extra)

    @classmethod
    def hello_world(cls):
        debug('Hello, World!')
        debug('Hello, World!', extra=cls.debug_extra)
        if DEBUG_ENABLED:
            cls.debug_dump()

DebugExample.setup()
DebugExample.hello_world()

Example output (sent to stdout):

debug: 02338ms:SConstruct:hello_world#90: Hello, World!
debug: 02339ms:SConstruct:DebugExample.hello_world#91: Hello, World!
debug: 02339ms:SConstruct:DebugExample.debug_dump#86: info: key=1, val=One
debug: 02339ms:SConstruct:DebugExample.debug_dump#86: info: key=2, val=Two
debug: 02339ms:SConstruct:DebugExample.debug_dump#86: info: key=3, val=Three

The third in a sequence of smaller PRs from #4409.

Internal changes only: no documentation or test updates necessary.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation

[bdbaddog]

What's going to use this?
Will this change what's output in the SCONS_MSCOMMON_DEBUG ?

[jcbrill]

What's going to use this?
Will this change what's output in the SCONS_MSCOMMON_DEBUG ?

When debug messages are issued from a class method (static or instance) the class name is prefixed to the method name.

Same debug statements with and without extra argument from classmethod function.

debug: 02338ms:SConstruct:hello_world#90: Hello, World!                <- without classname
debug: 02339ms:SConstruct:DebugExample.hello_world#91: Hello, World!   <- with classname

The WIP code changes have a non-trivial number of class/instance method debug calls. It simply provides more information in the debug log.