[4/n] [sled-agent] add code to read mupdate overrides

[sunshowers]

Don't make any decisions based on this at the moment -- simply read it for now. Decisions will happen in upcoming commits.

The logic here became quite complex, including tests, and I'm wondering if it would make sense to extract this into something more generic now or in the future.

Depends on:

• [3/n] [installinator] write out mupdate-override.json #8042
• [1/n] [sled-agent] move zone image source resolver into a new crate #8079
• [2/n] [sled-agent] put zone image resolver behind Arc/Mutex #8080

[sunshowers]

some review notes:

• this is a pretty big one, though it's like 65% tests
• I feel like we should extract this out into a common module at some point -- it feels like a somewhat different use case from Ledgerable though still related to it