prototype conversion to new dropshot builder interface

[davepacheco]

As part of oxidecomputer/dropshot#1122, although that should be a non-breaking change, I wanted to test-convert all of Omicron over to the new thing to make sure I didn't run into any unexpected issues. This worked out pretty well (and there were no callers that didn't immediately invoke start() on the HttpServerStarter).

This won't be able to land until:

• oxidecmoputer/dropshot#1122 lands
• we release a new dropshot version
• I update this PR to use the released version

[davepacheco]

For the record, this can't run in CI due to the local patch, but cargo xtask clippy and cargo nextest run --no-fail-fast were clean on commit c0ea923.