End to end reactivity for Seed resources

[petersutter]

What this PR does / why we need it:
This PR makes the seed resource reactive so that we receive updates via websocket connection.
In the shoot list, you can now see the conditions of the corresponding seed cluster in the Seed Readiness column. For now, this is visible only for gardener operators.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Real-time updates for seeds are now supported. In the shoot list, you can now see the conditions of the corresponding seed cluster in the `Seed Readiness` column. The `Seed` column has also been moved close to the newly introduced `Seed Readiness` column.

[gardener-robot]

@holgerkoser, @klocke-io, @grolu You have pull request review open invite, please check