Download buildtrees on demand for bst shell --build

Background

Enhance bst shell --build

Add a option to --use-buildtree that is like try but if the buildtree is in a remote cache but not the local cache then Buildstream will go get it.

This functionality could also be added to always so it only fails if it cant download the build tree.

Note:

Other code that try to get buildtrees are are push, workspace open and this.

As these are added we we need to make sure we do not have duplicate code paths or inconsistent behaviour.

Task description

• Add CLI option to --use-buildtree
• Add code to go get buildtree, this exists but might need making more genericl
• Logic code in _stream.py

Acceptance Criteria

A user can run bst shell --build --use-buildtree NewOption and have a buildtree downloaded and then used.

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