Add infrastructure ConfigValidator for checking NAT IPs

[stoyanr]

How to categorize this PR?

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

What this PR does / why we need it:

• Adds an implementation of the infrastructure.ConfigValidator for checking whether NAT IP names refer to existing and available external IP addresses. 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 #309

Special notes for your reviewer:

Release note:

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

[gardener-robot]

@stoyanr Labels area/todo, kind/todo do not exist.

[stoyanr]

/invite @ialidzhikov

[stoyanr]

@kon-angelo Thanks for your comments, addressed them and also added some unit tests.

[stoyanr]

/squash

[stoyanr]

/test

[testmachinery]

Testrun: e2e-7mwxl
Workflow: e2e-7mwxl-wf
Phase: Succeeded

+---------------------+---------------------+-----------+----------+
|        NAME         |        STEP         |   PHASE   | DURATION |
+---------------------+---------------------+-----------+----------+
| infrastructure-test | infrastructure-test | Succeeded | 9m45s    |
+---------------------+---------------------+-----------+----------+

[stoyanr]

/test

[testmachinery]

Testrun: e2e-vvlzm
Workflow: e2e-vvlzm-wf
Phase: Succeeded

+---------------------+---------------------+-----------+----------+
|        NAME         |        STEP         |   PHASE   | DURATION |
+---------------------+---------------------+-----------+----------+
| infrastructure-test | infrastructure-test | Succeeded | 10m30s   |
+---------------------+---------------------+-----------+----------+

[rfranzke]

/lgtm

[dkistner]

/lgtm