mavenBuild: add the possibility to define stash groups need to be stashed/unstashed

[vstarostin]

Changes

This PR adds the possibility of defining stash groups that need to be unstashed into container. The default value is 'source' to keep existing behavior and not break existing pipelines. We are not adding any default stash group to not change existing behavior and not break existing pipelines.

To set custom stash groups need to be unstashed into container, please set:

steps:
   mavenBuild:
      stashContent: ['source', 'git']
      ...

This PR is pretty much the same as #2271 (#2285) for npmExecuteScripts step.

• Tests
• Documentation

[vstarostin]

/it-go

[CCFenner]

Where was this unstashed before? Is there something we need to remove?

[vstarostin]

Where was this unstashed before? Is there something we need to remove?

@CCFenner No, nothing needs to be removed.
The source stash group was created at the very end of Init stage, then unstashing of source is happening at the beginning of the build stage. Unstashed files are used by steps of build stage.
Then mavenBuild creates the k8s pod and unstashes source directly into the pod’s filesystem.

[zomp1337]

@vstarostin is this PR going to be merged today?

[vstarostin]

/it-go

[sonarqubecloud]

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[vstarostin]

@CCFenner, @anilkeshav27 FYI: I removed the default source stash group to not change existing behaviour and not break existing pipelines (df4f9c1).