POC: Refactoring - moving seed determiner logic into dedicated struct

[tobiscr]

How to categorize this PR?

/area control-plane

What this PR does / why we need it:
We have a feature-request on Kyma side to offer customers an option to enforce that a seed and shoot are running in the same region. Our goal is to validate whether a seed would be available in a region before creating a shoot. This saves processing time on Gardener side and allows short roundtrip-times on our side in case of non-matching seed clusters.

This PR is a pure refactoring of the Gardener reconciler which is moving logic related to determine the seed into a separate struct. No functional changes were applied, just code re-arrangements. The introduced SeedDeterminer is also visible outside of ths shoot-package which would allow us to reuse the logic in our application.

The change improves also a bit the separation of concerns within the logic. The reconciler is purely responsible for reconciliations, the seed-determiner does nothing beside determining the seed cluster.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Release note:

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[tobiscr]

After a call with @ScheererJ we agreed on following options for the seed determining logic:

1. We change the visibility of the determineSeed function (from determineSeed to DetermineSeed) on the current Reconciler to make it public accessible and consumable for other applications (challenge could be to set the right strategy for detecting a suitable seed as it's depending on the Shoot purpose)
2. We consider the proposed PR and extract the determineSeed logic into a new struct (challenge could be to set the right strategy for detecting a suitable seed as it's depending on the Shoot purpose)
3. The other application implements their own determining logic by checking for the existence of a seed in this region

@ScheererJ is getting in contact with a team colleague to discuss these options and will reply in the coming 1-2 weeks with a suggestion how to proceed.

[ScheererJ]

After a call with @ScheererJ we agreed on following options for the seed determining logic:

1. We change the visibility of the determineSeed function (from determineSeed to DetermineSeed) on the current Reconciler to make it public accessible and consumable for other applications (challenge could be to set the right strategy for detecting a suitable seed as it's depending on the Shoot purpose)
2. We consider the proposed PR and extract the determineSeed logic into a new struct (challenge could be to set the right strategy for detecting a suitable seed as it's depending on the Shoot purpose)
3. The other application implements their own determining logic by checking for the existence of a seed in this region

@ScheererJ is getting in contact with a team colleague to discuss these options and will reply in the coming 1-2 weeks with a suggestion how to proceed.

After offline discussion, option 1 seems to be a simple and acceptable solution. Please adapt the pull request accordingly.

Please note that a general option to enforce same region seed clusters in scheduling may be something for a future gardener hackathon. Let me know if you would like to join :-) .

[ScheererJ]

/assign

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PR needs rebase.

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[gardener-ci-robot]

The Gardener project currently lacks enough active contributors to adequately respond to all PRs.
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[gardener-prow]

@gardener-ci-robot: Closed this PR.

In response to this:

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