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 (Add feature flag to drop dependencies from test execution #571)
• Next phase of integration with the colcon-python-project prototype

[nuclearsandwich]

I think this is handy for introducing changes in an otherwise "stable" CLI. I've left an implementation comment.

[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]

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 ⚠️

Additional details and impacted files

@@            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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