BM-615 - Add caching for assessor and set builder images (#393)

Closes #388 

Uses reqwest middleware to add a HTTP-aware cache. See
https://docs.rs/http-cache-reqwest/latest/http_cache_reqwest/index.html.
This is used only for caching requests for assessor and set-builder
images at this time.

- Adds caching to requests to retrieve assessor and set builder images
- - Can trivially also add it to user images but this may need more
discussion
- Add new command line flag `--cache-images` to broker
- Adds new command line arg `--cache-dir` to broker (default
`/tmp/broker_cache/`)

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Co-authored-by: Austin Abell <[email protected]>
Co-authored-by: Cohan <[email protected]>
Co-authored-by: Sasha <[email protected]>
Co-authored-by: pote.eth <[email protected]>
Co-authored-by: Parker Thompson <[email protected]>
Co-authored-by: Victor Graf <[email protected]>
Co-authored-by: Rami Lukata <[email protected]>
Co-authored-by: Richard Howard <[email protected]>
Co-authored-by: Angelo Capossele <[email protected]>

Author: 10 people