docs: managementv3 cluster support strategy

[salasberryfin]

What this PR does / why we need it:

This ADR is a proposal on how support for management.cattle.io/v3 clusters is implemented, based on investigation during the development process.

The main point of concern here is how we manage cluster naming. Since management.cattle.io/v3 clusters are required to have a name that satisfies the string c- followed by exactly five characters, we cannot use the display name (the one that appears in Rancher dashboard) as we do with provisioning.cattle.io/v1 clusters (where name and display name are the same).

Creating a randomized 5-character suffix presents the challenge of not being able to track the resource while reconciling like we do in the existing provisioning.cattle.io/v1 import controller, since we fetch the cluster on Name and Namespace (refer to here).

A reasonable alternative is to compute the SHA256 hash of the display name and then truncate the result to fit in the 5-character prefix of the name. This would make it easy and cheap to deterministically get the resource name based on the display name. The problem with this approach is that we could potentially encounter collisions if the number of clusters is large.

We will be tracking the cluster based on two new labels that will be applied to all management.cattle.io/v3 Rancher clusters that uniquely reference the CAPI cluster that owns the resource. Using this approach, we can leverage the built-in functionality of Kubernetes by passing the field GenerateName with the prefix c- so that it randomly generates a 5 character suffix that satisfies the regular expression, and retries if a name collision occurs.

Which issue(s) this PR fixes:
Fixes #361

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[salasberryfin]

I updated the ADR document and the PR description based on the feedback. Thanks @Danil-Grigorev @richardcase.

[salasberryfin]

Thanks for the comments and reviews folks.