Add central interface for defining feature flags

[cottsay]

The intended use for colcon feature flags is to ship pre-production and prototype features in a disabled state, which can be enabled by specifying a particular environment variable value. By using an environment variable, these possibly dangerous or unstable features are hidden from common users but are enabled in a way which can be audited.

Current examples of possible uses:

• Dropping dependencies from execution during colcon test (#571)
• Next phase of integration with the colcon-python-project prototype

[cottsay]

Also discussed offline: The is_feature_flag_set now logs a warning the first time an enabled feature is queried. It will look like this on the console:

[0.208s] colcon.colcon_core.feature_flags WARNING Enabling feature: foobar

I'd rather not directly write to stdout or stderr, and INFO level log messages are not printed to the console by default, so this seemed like the best move. A warning also highlights the possible instability of the feature's use.

[nuclearsandwich]

I'd rather not directly write to stdout or stderr, and INFO level log messages are not printed to the console by default, so this seemed like the best move. A warning also highlights the possible instability of the feature's use.

I think warning is appropriate since feature flags are used to change default behavior in inherently unstable and under-tested ways (i.e. we will never test with the complete cartesian product of feature flag values).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 83.33%. Comparing base (485f500) to head (6edd2b5). Report is 1 commits behind head on master.

Files	Patch %	Lines
colcon_core/feature_flags.py	96.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #640 +/- ## ========================================== + Coverage 83.24% 83.33% +0.09% ========================================== Files 66 67 +1 Lines 3850 3877 +27 Branches 760 766 +6 ========================================== + Hits 3205 3231 +26 Misses 556 556 - Partials 89 90 +1 ```

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