WIP: Make uploading of build trees configurable

Description

WIP: Make uploading of build trees configurable

Changes proposed in this merge request:

• Add a new config option to artifact cache config to specify pushing artifacts without their buildtrees, supporting if it is or isn't cached locally. Multiple projects can define the same remote url, whilst being able to define if their respective elements should be pushed partially or not
• Servers can now support caching 'partial' artifacts, these being artifacts with digests to buildtrees that are not stored
• Tests to ensure this behaviour is supported, in a number of scenarios

This merge request, when approved, will close: #566

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