feat: add custom User-Agent for Nutanix API requests (fixes #508)

[Himanshucodess]

What does this PR do?

Implements custom User-Agent for Nutanix API requests as per #508.

• Adds pkg/version with GitCommit and UserAgent helper (capx/, fallback dev).
• Sets User-Agent on prism-go-client v3 cache via WithUserAgent in controllers/helpers.go.
• Updates Makefile ldflags to propagate GIT_COMMIT_HASH to both main.gitCommitHash and pkg/version.GitCommit.

Filtering is client-side via existing SDK header mechanism; no undocumented API behavior introduced.

Why is this needed?

Improves observability and traceability in Nutanix API logs/metrics, facilitates debugging/support, and aligns with API best practices. Previously requests used generic SDK default (nutanix/v3).

Testing

• go test ./pkg/version -v -> PASS (2 tests)
• go test ./pkg/client -v -> PASS
• go vet ./pkg/version ./pkg/client ./controllers -> PASS
• gofmt -> PASS
• E2E not run (requires Nutanix Prism Central)

Notes

• Only v3 client has WithUserAgent in prism-go-client v0.8.2; v4 and converged clients use default until upstream adds support.
• Version is git commit hash via ldflags, fallback dev if not set.
• No breaking changes, no auth/TLS/proxy behavior change.

Fixes #508