Configuration option for disabling blob fetching with RE

Background

A follow up to #832 (closed) as brought up here, this would allow users to configure BuildStream to stop fetching blobs in more scenarios where they're using RE, for the actually build targets and the run time dependencies. This can also be extended to sources, as if these are in the remote source cache, we only need the directory trees in order to compose these together.

Task description

• Add configuration option to stop BuildStream from fetching blobs
• Change how artifacts are pulled to not pull actual file blobs if this option is set
• Change how sources are pulled similarly

Acceptance Criteria

Remote execution should continue to work but without file blobs being kept locally where possible. Some extra tests should be added to check this is working as intended.

---