Implement a cert watcher to use new certificates without restarting

[black-dragon74]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This PR implements a CertWatcher which is responsible for watching the TLS Cert and Key files.

It also sets a callback GetCertificate which if set in the gRPC TLS opts, will be called every time a certificate data is required, enabling hot reloads and removing the requirement to restart the sidecar.

Which issue(s) this PR fixes:

Fixes #152

Special notes for your reviewer:
NONE

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

Hi @black-dragon74. Thanks for your PR.

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[Rakshith-R]

/ok-to-test

[xing-yang]

/assign

[iPraveenParihar]

Integration tests are failing because readinessProbe for csi-snapshot-metadata sidecar is failing.
Sent a fix kubernetes-csi/csi-driver-host-path#624

cc @carlbraganza @xing-yang

[kaovilai]

fyi kubernetes-csi/csi-driver-host-path#624 merged with v0.2.0 release which does not contain the http health probe from #190 .

[Rakshith-R]

fyi kubernetes-csi/csi-driver-host-path#624 merged with v0.2.0 release which does not contain the http health probe from #190 .

https://github.com/kubernetes-csi/external-snapshot-metadata/actions/runs/20814019470/job/60361307710?pr=196
The integration test overrides the sidecar image with the one built from this pr.
We'll get these changes in and do a v0.3.0 release and update in hostpath.

[Rakshith-R]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: black-dragon74, Rakshith-R

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