Request Tree children by SHA rather than path
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### Release notes
* Added API endpoint to retrieve children of a tree node by id rather than path
### Problem to solve
I want to be able to request the children of a tree based on the tree's id (or sha?)
### Proposal
`/repository/tree?sha=jas98dfn21` to request the children of a tree rather than using the tree's path
### Intended users
Developers using the Gitlab API
Personas are described at https://handbook.gitlab.com/handbook/product/personas/
* [Priyanka (Platform Engineer)](https://handbook.gitlab.com/handbook/product/personas/#priyanka-platform-engineer)
I believe the Github API only allows requesting by SHA/ID
issue