Include HTTP version (1.1/2/3) in the spec

[rafalkrupinski]

The servers are typically backwards compatible so any API client could connect using the lowest version,
but including HTTP versions in the spec would tell a client developer that they can use a special library and connect via the higher version.

I propose something like this:

openapi:
    http-versions:
    - '1.0'
    - '1.1'
    - '2.0'
    - '3.0'

[handrews]

This is being discussed for Moonwalk (4.0):

• Add support for newer HTTP versions (2/3) sig-moonwalk#45

Note that moonwalk features will be backported to 3.x when feasible, so it's not necessary to track in both places - closing here in favor of the moonwalk discussion which has more comments as well.