Tag config values that should be inherited similar to how plugin config works

[Tatskaari]

I've allowed plugin authors to define whether some config fields should be inherited from the host repo in #2216

We should do something similar for the non-plugin config values. This is a little more complex in cases like BUILD_FILE_NAME, where we might define that in the host repo. This is set by the core.DefaultConfiguration() method, so in a naive implementation, we'd read the config value from the subrepo's .plzconfig where it hasn't been set. In this case we'd end up with no build file names configured.

We may also want to somehow control which config values get inherited for config that is used outside the CONFIG dict. This needs some design work and perhaps it would be better to handle this when reading config into the Configuration struct.

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