Remove GW API dependency

[kfswain]

Currently we rely on the GW API for the conformance test suite. Recently, some changes to the upstream GW API conformance tests caused breaking changes in the conformance tests. This impacts gateways that run the up-to-date GW API conformance tests & the IGW conformance tests.
Context: #1691 (comment)

We should remove our GW API dependency as a little copying is better than a little dependency: https://go-proverbs.github.io/ (thanks @nirrozenbaum for the reminder!). And Allow GW API to operate & maintain their conformance tests as they see fit, independently from us.

CC @zetxqx @robscott

[nirrozenbaum]

+++

[zetxqx]

/assign

I'll see if there is some easier way to do this.