gep-1762: Add attachTo to in cluster deployments GEP

[howardjohn]

What type of PR is this?
/kind gep

What this PR does / why we need it:

This is a followup to #1757. This was stripped out to get things into 1.0

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

A new `spec.infrastructure.attachTo` field has been added to `Gateway` to managed manually deployed infrastructure.

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[robscott]

This feels awfully similar to #1863 - it seems like we shouldn't try to tackle one of the problems (Gateway merging) without the other (Gateway Service attachment) given the significant overlap. Unfortunately I think we've run out of time in the v1.1 release cycle to fit something this big in, but I'd like to aim to get both concepts in as one of our first topics post v1.1 release.