docs: Gateway implementation guide for managed vs. unmanaged gateway lifecycles and intra vs extra cluster

[shaneutt]

What would you like to be added:

This is a request for documentation for implementations which would provide additional guidance and prescription on how to implement Gateway controllers for different kinds of underlying Gateway infrastructure options. Particularly the guide should cover:

• Gateways which are "unmanaged" and refer to an existing deployment

Additional Goals:

• define clear terminology about the underlying lifecycle management and network location of Gateways

Why this is needed:

This is a direct follow up from a community meeting where the overall topic was discussed, and was a follow up to:

• @howardjohn's Document on the subject
• A related discussion which suggested some implementation details for how to solve some of the problems in this domain with spec changes: Gateway: Administrative Endpoints #877

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/lifecycle stale

[shaneutt]

/remove-lifecycle stale

[shaneutt]

I've gone through a couple drafts that I have reviewed with peers on this one. I've always found my own results wanting. The fact of the matter is that trying to implement for unmanaged mode is difficult, and it's problematic to provide any prescription for others to do the same since it gets deep into implementation-specific territory for each implementor.

I'm also convinced now that the value of writing this guide is pretty limited since we expect that most implementors are going to be able to implement Gateway in a native way, and my team is even considering not releasing the unmanaged way. My main suggestion to anyone reading this who might actually be looking for guidance is to ensure their implementation passes the conformance tests. Since I created this, I am going to close it for now. If anyone is looking to work on this I'll be happy to re-open it.