Helm: allow vsphere.conf to be sourced from hostPath

[skools-here]

What this PR does / why we need it:
Adds an optional Helm configuration to allow vsphere.conf to be sourced from a hostPath volume.

This is useful for bootstrap and constrained environments where the configuration file already exists on the node and creating a ConfigMap/Secret is not desirable. The default behavior using a ConfigMap remains unchanged.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #1460

Special notes for your reviewer:
The change is fully backward compatible (config.source=configMap remains the default).

When config.source=hostPath, the ConfigMap is not created and the DaemonSet mounts the file directly from the node filesystem.

Helm validation is included to prevent misconfiguration.

Release note:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: skools-here
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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @skools-here. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[skools-here]

Hey @chenlin07 , could you please take a look at this PR?
Kindly review it and let me know if any changes are needed..

[zhanggbj]

/ok-to-test

[zhanggbj]

/test pull-cloud-provider-vsphere-e2e-test-on-latest-k8s-version