tools: move lister and verifier to tools/

[iPraveenParihar]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change

/kind bug

/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

Problem: Users cannot install the snapshot-metadata-lister and snapshot-metadata-verifier tools using go install due to the main module's replace directives:

go install github.com/kubernetes-csi/external-snapshot-metadata/examples/snapshot-metadata-verifier@v0.2.0

Results in:

go: The go.mod file for the module providing named packages contains one or
    more replace directives. It must not contain directives that would cause
    it to be interpreted differently than if it were the main module.

Solution: Extract the tools into separate Go modules under tools/ directory. Each tool now has its own go.mod without any replace directives.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

The snapshot-metadata-lister and snapshot-metadata-verifier tools can now be installed directly using `go install`:
- `go install github.com/kubernetes-csi/external-snapshot-metadata/tools/snapshot-metadata-lister@latest`
- `go install github.com/kubernetes-csi/external-snapshot-metadata/tools/snapshot-metadata-verifier@latest`

Previously, installation failed due to `replace` directives in the main module's go.mod.

[k8s-ci-robot]

Hi @iPraveenParihar. Thanks for your PR.

I'm waiting for a kubernetes-csi 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[Rakshith-R]

/ok-to-test

[iPraveenParihar]

Integration tests failing because of certs expired, fixed by #203

[PrasadG193]

Can we keep the tools in examples repo?

[kaovilai]

symlinks?

[iPraveenParihar]

Can we keep the tools in examples repo?

we will end up with ambiguous import because Go finds the same package path in module cache: When examples/snapshot-metadata-verifier/go.mod requires github.com/kubernetes-csi/external-snapshot-metadata v0.2.0.

Since the published v0.2.0 module doesn't have a tools/ directory, there's no conflict, Go can only find it from the local go.mod.

[Rakshith-R]

This change is required to complete k8s e2e suite
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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