Enable consumers of gitops-engine API to specify an alternative temp directory for generated manifests, as an alternative '/dev/shm'

[jgwest]

See parent issue for details: #614

This PR:

• When calling gitops-engine, you will now need to explicitly define the temporary path to write kubectl manifests to:
  • Add new parameter tmpManifestPath to public APIs:
    • NewEngine in pkg/engine/engine.go
    • NewClusterCache in `pkg/cache/cluster.go
  • In both cases, when calling these functions, you now must specify where to store temporary manifest files that are generated
  • If you wish to preserve the existing behaviour, just use the value from calling io.TempPathUseDevShmIfAvailable() (see examples in this PR)
• Updates existing gitops-engine code to use new API
• Preserves the existing behaviour of the code: code and tests that were previously using /dev/shm will continue to do so
• Updates pkg/utils/io/io.go API to make it more explicit about behaviour

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[ppapapetrou76]

The gitops-engine repository is migrating to https://github.com/argoproj/argo-cd. We are closing all draft PRs.
Please feel free to submit it again once the migration is over.