Fetch GCP Subnetwork from API

[kgroschoff]

What this PR does / why we need it:
Fetch GCP Subnetworks from API.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Related to https://github.com/kubermatic/dashboard-v2/issues/1519

Special notes for your reviewer:
/hold
until kubermatic/kubermatic#4997 gets merged

Release note:

Fetch GCP Subnetworks from API

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kgroschoff

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Needs approval from an approver in each of these files:

• OWNERS [kgroschoff]

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[codecov]

Codecov Report

Merging #1950 into master will decrease coverage by 0.26%.
The diff coverage is 21.66%.

@@            Coverage Diff             @@
##           master    #1950      +/-   ##
==========================================
- Coverage   69.28%   69.02%   -0.27%     
==========================================
  Files         246      246              
  Lines        8931     9055     +124     
  Branches     1124     1162      +38     
==========================================
+ Hits         6188     6250      +62     
- Misses       2737     2799      +62     
  Partials        6        6

Impacted Files	Coverage Δ
...provider-options/gcp-provider-options.component.ts	30.57% <18.6%> (-6.61%)	⬇️
src/app/core/services/api/api.service.ts	35.75% <25%> (-0.27%)	⬇️
src/app/core/services/wizard/provider/gcp.ts	55.76% <27.27%> (-7.65%)	⬇️
src/app/testing/services/api-mock.service.ts	68.14% <50%> (-0.33%)	⬇️
...uster/cluster-details/cluster-details.component.ts	68.84% <0%> (+2.51%)	⬆️

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[maciaszczykm]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 30a387daf9ca0e1726682899a681121f89dbf047

[kgroschoff]

/hold cancel

[kgroschoff]

/test pull-dashboard-e2e