Trigger control plane migration

[grolu]

What this PR does / why we need it:
This PR adds the ability to trigger control plane migration from the gardener dashboard.
The dialog is only visible to users that have permission to patch the shoots/binding subresource.

Which issue(s) this PR fixes:
Fixes #1256

Special notes for your reviewer:

Release note:

Operators can now trigger control plane migration via a configuration dialog. The configuration button is hidden for regular users, unless they have been granted permission to trigger a control plane migration for their shoots

[grolu]

@plkokanov @RaphaelVogel

[plkokanov]

Thanks a lot!
Just a few questions:

• It looks like the seed cluster selection is a drop down dialogue. Does it list all seeds on the garden landscape or only those that match the shoot's provider? I guess users will also require permissions to list seeds.
• Should we rename the name of the dialogue to Change Seed instead of Configure Seed? At least to me it seems a bit more appropriate.

[grolu]

It looks like the seed cluster selection is a drop down dialogue. Does it list all seeds on the garden landscape or only those that match the shoot's provider? I guess users will also require permissions to list seeds.

Yes, we currently do not filter by provider. We can add that.
I tested with a service account, I was able to list the seed using the service account and only needed to add a role to patch the /binding subresource

Should we rename the name of the dialogue to Change Seed instead of Configure Seed? At least to me it seems a bit more appropriate.

We can rename it I labelled it that way because we have it like that for all the Configure Dialogs, e.g. purpose is also called Configure Purpose so I guess we should change it for hat one, too?

[petersutter]

/lgtm