Add transforms for index routes

[ahal]

We have the concept of "index_builders" in the task.py transforms, but:

1. These are Mercurial specific (see Generic 'index_builder' is Mercurial specific #24 )
2. They are buried in the big sea of functionality in task.py

I think it would make sense to have a dedicated transform that can add index routes based on the context. E.g it could support routes for:

• push
• pull-request
• release promotion
• ???

Ideally this transform would help us be more consistent with our index routes across projects.