Add context-keyword notify feature

[jridgewell]

Context: ampproject/amphtml#28203

When an extension adds prerenderAllowed to a JS file, it then requires approval by Runtime team. It'd be nice if we could add a context-keyword setting to the notify settings, so that this is done automatically.

[rcebulko]

While I can see how this would definitely be a useful feature, it doesn't seem to have a close tie-in with most of the owners bot functionality. Are there many other cases like this where particular tokens require team approval, or is it more typically a set of runtime-related keywords? I ask because my hunch is that this may be better solved with a dedicated app to scan each PR and block where relevant.