factor-outbound-http: Refactor wasi impls

[lann]

Putting my commits where my fingers type.

This is split into 3 5 commits to ease review:

• A small test change trying to squash a persistent flake
• Reorganizing send_request_impl, inlining send_request_handler and making it a method on a new struct that holds most of its inputs. This doesn't do anything functionally but I think it helps distinguish the data that was copied from the instance state and may aid future refactoring.
• Reordering some top level items in the file to better fit (what I think of as) a common convention, roughly "things higher in the file call into things lower in the file".
• Merging the separate TLS_CLIENT_CONFIG task-local into CONNECT_OPTIONS, makes connect_tcp and connect_tls methods on ConnectOptions, and makes a bunch of miscellaneous simplifications (which I now realize could/should have done in a fourth commit; sorry!).
• Another refactor splitting the send method into a few parts to help avoid confusion around error handling.

[lann]

Bisecting, the test failure surprisingly happens in the first commit. 🤔

[lann]

OK I'm pretty sure this is just a subtle change to the way errors got propagated with the nested Results.

[lann]

Squashing that bug inspired/compelled me to do another round of refactoring to get rid of the nested Results.

[lann]

The last CI run succeeded; I just needed to sign my commits.