GEP-1651: Gateway Routability (#1653)

* Initial GEP draft with goals/non-goals, intro and references

* Update GEP with an suggested implementation

* fix typo

* Move GEP to new folder

* Incorporate discussion into GEP

* Review feedback
- move scope to gateway.spec.infrastructure
- rename scope to reachability

* specify that some status.address semantics

* address feedback

* address gatewayclass vs. gateway

* fix spelling

* more reasons to allow Gateways to specify reachability

* Alternative: Adding attributes to a GatewayClass

* fix headings

* add a section about mutability

* add envoy gateway to references

* Fix some wording to address comments

#1653 (comment)
#1653 (comment)

* update constants used in goals to match the API

* reachability => routability

* Use Private/Public antonyms - update goals to be inclusive new routable options

* add routability to GatewayAddress and define some semantics

* fix formatting

* introduce GatewayStatusAddress

* fix attribute name

* GatewayClass status now includes details about routability

* grammar change

* if mutation can't be prevented then incompatible routability should set conditions to false

* Change Invalid reason to UnsupportedRoutability

* Use a reason UnsupportedRoutability if the Gateway doesn't support it

* add a section about in-cluster gateway not support private routability

* fix indentation

* Annotate types with godoc

* fix formatting/linking

* drop Ready condition and allow old gateways to be present if mutability fails

* add to nav