Add infrastructure ConfigValidator for checking floating pool names

[stoyanr]

How to categorize this PR?

/area ops-productivity
/area quality
/kind enhancement
/platform gcp

What this PR does / why we need it:

• Adds an implementation of the infrastructure.ConfigValidator for checking floating pool names. See Add infrastructure config validator gardener#4547.
• Adds unit tests for the infrastructure.ConfigValidator implementation.
• Vendors g/g v1.31.0-dev (in a separate commit).

Which issue(s) this PR fixes:
Fixes #328

Special notes for your reviewer:

Release note:

Floating pool names in infrastructure config are now checked if they exist, and if not the issue is properly reported as `ERR_CONFIGURATION_PROBLEM` with a clear error message.

[stoyanr]

/invite @ialidzhikov @rfranzke @dkistner

[stoyanr]

/squash
/test

[testmachinery]

🔥 Oops, something went wrong @stoyanr
.
Tip: Check whether the test is correctly defined by flags or by the default test config in the repository.

Command test
Runs a testrun that is either specified by command line flags or in the default values.
The specified path is rendered as testrun and the current repository is injected as a default location.


Example: /test [sub-command] [--flags]

Usage:
      --dry-run              Print the rendered testrun
      --set stringArray      sets additional helm values
      --template             run go templating on the configured file before execution
      --testrunPath string   path to the testrun file that should be executed

Instructions for interacting with me using PR comments are available here

[stoyanr]

/test

[testmachinery]

🔥 Oops, something went wrong @stoyanr
.
Tip: Check whether the test is correctly defined by flags or by the default test config in the repository.

Command test
Runs a testrun that is either specified by command line flags or in the default values.
The specified path is rendered as testrun and the current repository is injected as a default location.


Example: /test [sub-command] [--flags]

Usage:
      --dry-run              Print the rendered testrun
      --set stringArray      sets additional helm values
      --template             run go templating on the configured file before execution
      --testrunPath string   path to the testrun file that should be executed

Instructions for interacting with me using PR comments are available here

[rfranzke]

/lgtm

[dkistner]

/lgtm