Allow users to control which content types are included in
model generation for requests, responses.

Users can now fully control the struct tags that are generated, with
default values of json and form encoding

Generated code must have a header saying it's generated.

Simplify names of schemas discovered by traversing the paths element
of the spec.

Ensure that we're fixing existing open issues.

Recreate old issue tests to make sure everything still works
as expected.

Add a Nullable type and use it where explicitly requested.

Add extension properties which match the old oapi-codegen behavior, and
fix a bunch of related issues.

Parameter binding and styling will now be embedded into the generated
code, and not rely on an external runtime package.

Add first server support, using built in http, and refine templates
to make it work. Example perstore implementation.

Models can be imported into clients now, so they can live in
a separate package.

Clean up example client to use the models import, and clean up
tests.

middleware and validation

Libopenapi now supports parsing the model without resolving
external references, which is exactly what we wanted.

Add webhook and callback support, and show examples of how to
use them.

This was an unnecessary complication and we'll cross this bridge if
we ever get to it.

Also, fix lint issues.

echo V5 needs go 1.25, so we can only validate it if
running the 1.25 toolchain