add format overriding by environment variables

[SammyRamone]

This is a proposed fix for ros2/launch_ros#368
Originally, I planned on reusing the RCUTILS_CONSOLE_OUTPUT_FORMAT environment variable for this. Hoewever, this is not possible since launch and rcutils rely on two different logging implementations which have different names for the same things. For example one supports {line_number} and the other {lineno}. Instead I added two environment variables OVERRIDE_LAUNCH_SCREEN_FORMAT and OVERRIDE_LAUNCH_LOG_FORMAT since screen and log formatting is handled differently in the code.
I also framed the variables as override similar to #713

Any feedback is welcome.

[SammyRamone]

friendly ping @adityapande-1995 @methylDragon @wjwwood

[methylDragon]

Heya, thanks for the contribution! I'm not in principle opposed to this, but could you add a test to the logging test for this? That would help prevent regressions!

https://github.com/ros2/launch/blob/cef1ce3217cd39e339bbaa8c62d8868392fbcba0/launch/test/launch/test_logging.py

[SammyRamone]

Sorry for the long wait. This was fairly low-priority for me and I only now had time to work on it again.
I added a test for the environment variables and also ensured that the other tests don't fail if the environment variable is set in the terminal in which the tests are executed.

[SammyRamone]

I also added a draft PR for documenting this PR ros2/ros2_documentation#4061

[SammyRamone]

friendly ping @methylDragon

[adityapande-1995]

@SammyRamone thanks for the patience ! The team had a discussion about this, and while we do think there should be a holistic way to configure all of this logging, this is a good feature. I'll run full CI on this.

[adityapande-1995]

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