API for Default Gateways (#3887)

* Add API to GEP-3793.

Signed-off-by: Flynn <[email protected]>

* Minor fixes in text, more important fixes in list formatting.

Signed-off-by: Flynn <[email protected]>

* Major update for multiple default Gateways, including clarification of goals and non-goals as needed and acknowledgment that a defaulted Route gives up some control.

Signed-off-by: Flynn <[email protected]>

* Rearrange Prior Art based on feedback; also split the explicit/implicit bit into its own paragraph.

Signed-off-by: Flynn <[email protected]>

* 🤦‍♂️ I can count, really.

Signed-off-by: Flynn <[email protected]>

* Massive update for `defaultOK: true` instead of "no parentRefs".

Signed-off-by: Flynn <[email protected]>

* Use `spec.defaultOK` to parallel `spec.isDefault`.

Signed-off-by: Flynn <[email protected]>

* Minor tweaks for feedback.

Signed-off-by: Flynn <[email protected]>

* Mark as implementable

Signed-off-by: Flynn <[email protected]>

* Change `defaultOK` to `useDefaultGateway`.

Signed-off-by: Flynn <[email protected]>

* Move 3793 to Implementable in mkdocs.yml

Signed-off-by: Flynn <[email protected]>

* Define default "scopes", with the only currently-defined scope being All. Update isDefault to defaultScope; switch both it and useDefaultGateway to scopes instead of bools.

Signed-off-by: Flynn <[email protected]>

* Add RFC8174 definitions

Signed-off-by: Flynn <[email protected]>

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Signed-off-by: Flynn <[email protected]>

Author: kflynn