Condition -> Changesets should allow a custom variable

In pipelines, we now have condition changesets.includePaths

A step can run if the path matches. But the step has no idea which path is being matched.

It would be really helpful to have a keyword defined against each path and have it available in the ENV. Something like regex capture group.

For example, a monorepo wtih top level directories corresponding to different services. This will allow us to write something like this* and let the running step know which service to build/test/deploy.

```

condition:  changesets:    includePaths:      - "{user}/**"

```

In the step, something like ${BITBUCKET_CHANGESETS_PATH_CAPTURE_GROUP} which can have the value `user`.

This way, we can reuse the same stepdifinition for all services instead of coy pasting over and over again.