Add TransitionFactory interface, to be used later for creating

[katre]

transitions.

Part of work on #7814.

[gregestren]

Since, as I understand, we're going to funnel all transitions through this interface, even if they can be instantiated statically, can we provide more context for this javadoc?

Also, let's reference

bazel/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/PatchTransition.java

Lines 31 to 33 in 1f684e1

	* input build options and any immutable member fields. Implementations must also override
	* {@link Object#equals} and {@link Object#hashCode} unless exclusively accessed as
	* singletons. For example:

(the importance of resolving to singletons when possible, otherwise being careful to support .equals), lest we run into hard-to-track bugs when these get passed around hash maps, etc.

[katre]

Added more details and added that note.

I'm still not entirely sure if everything will be factories, or if we'll still have static transitions. I can come back and update this Javadoc once a decision becomes clear during the implementation.