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

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

argoproj / gitops-engine