Add SHA256 checksum verification and robust URL validation for image

[tsj-30]

What this PR does / why we need it:
This PR adds SHA-256 checksum verification and robust URL validation for image downloads in the convert-upload-images-powervs script.
It ensures that corrupted or partially downloaded .qcow2 images (caused by unescaped characters like & in URLs) are detected and the script terminates safely instead of continuing with invalid data.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #92

Special notes for your reviewer:

**Output/Demonstration

sjtharun@Ss-MacBook-Pro convert-upload-images-powervs % ./convert-upload-images-powervs \
  --rhel-url "https://cloud.centos.org/centos/10-stream/ppc64le/images/CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2" \
  --rhel-sha256 "27bdeb53a4e265409adcf37caa87103a5c544640fc99da19c06f6ff1dd389b30" \

+zsh:77> ./convert-upload-images-powervs --rhel-url https://cloud.centos.org/centos/10-stream/ppc64le/images/CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2 --rhel-sha256 27bdeb53a4e265409adcf37caa87103a5c544640fc99da19c06f6ff1dd389b30
LOG: =========================================
LOG: Starting download: CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2
LOG: Source URL: https://cloud.centos.org/centos/10-stream/ppc64le/images/CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2
LOG: =========================================
LOG: Download attempt 1 of 3
LOG: Downloading CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2...
LOG: Download completed, verifying integrity...
LOG: Verifying file size for CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2...
LOG: Expected size: 739220480 bytes
LOG: Actual size:   739220480 bytes
SUCCESS: ✓ File size verification PASSED
LOG: Verifying SHA256 checksum for CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2...
LOG: Expected: 27bdeb53a4e265409adcf37caa87103a5c544640fc99da19c06f6ff1dd389b30
LOG: Actual:   27bdeb53a4e265409adcf37caa87103a5c544640fc99da19c06f6ff1dd389b30
SUCCESS: ✓ Checksum verification PASSED for CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2
SUCCESS: =========================================
SUCCESS: ✓ Download and verification completed successfully!
SUCCESS:   File: CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2
SUCCESS:   Location: ./CentOS-Stream-GenericCloud-10-20241118.0.ppc64le.qcow2
SUCCESS: =========================================

[ppc64le-cloud-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tsj-30
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[ppc64le-cloud-bot]

Welcome @tsj-30! It looks like this is your first PR to ppc64le-cloud/pvsadm 🎉

[ppc64le-cloud-bot]

Hi @tsj-30. Thanks for your PR.

I'm waiting for a ppc64le-cloud 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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[mkumatag]

@sudeeshjohn anyone using this script? may be need your reviewing this script

[sudeeshjohn]

I would suggest to add the checksum validation in getImage function as well

pvsadm/cmd/image/qcow2ova/get-image.go

Line 34 in 1936990

func getImage(downloadDir string, srcUrl string, timeout time.Duration) (string, error) {

[kishen-v]

/ok-to-test