refactor: move config TOML parsing into a submodule

[gjcolombo]

(Part 2 in the instance spec refactoring series; see #735.)

Move large parts of the config TOML parsing logic in server spec builder into their own module. The parsing logic is functionally the same; the main difference is that there's a new, more descriptive error type (ConfigTomlError) that converts to a ServerSpecBuilderError variant instead of a variant that just takes a String. The idea is eventually to have separate modules for parsing config TOMLs, InstanceEnsureRequests, and VersionedInstanceSpecs; this is the first small step in that direction.

Tests: PHD, cargo test, ad hoc propolis-server testing; launched a Falcon topology and verified by hitting the /instance/spec endpoint that it created all the correct devices.

[hawkw]

overall, this looks good to me. i had some small suggestions about the code that was already here, but feel free to disregard me since that code was already here, and i understand not wanting to mess with it in a diff that mostly just moves things around.

[gjcolombo]

Will merge this and push part 3 first thing Monday. Thanks as always for the review, @hawkw!