Add Openapi spec and STUB endpoint for source_preview

[ppadti]

Description

This PR aims to add source_preview endpoint. Added types, mocks.

How Has This Been Tested?

run the application:
make dev-start

test the curl command:
for HF:

curl -X POST "http://localhost:4000/api/v1/settings/model_catalog/source_preview?namespace=kubeflow" \
  -H "Content-Type: application/json" \
  -H "kubeflow-userid: user@example.com" \
  -d '{
    "type": "huggingface"
  }' 

for yaml type:

curl -X POST "http://localhost:4000/api/v1/settings/model_catalog/source_preview?namespace=kubeflow" \
  -H "Content-Type: application/json" \
  -H "kubeflow-userid: user@example.com" \
  -d '{
    "type": "yaml"
  }'

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages
• Automated tests are provided as part of the PR for major new functionalities; testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.
• For first time contributors: Please reach out to the Reviewers to ensure all tests are being run, ensuring the label ok-to-test has been added to the PR.

If you have UI changes

• The developer has added tests or explained why testing cannot be added.
• Included any necessary screenshots or gifs if it was a UI change.
• Verify that UI/UX changes conform the UX guidelines for Kubeflow.

[mturley]

/lgtm
/approve

ah @ppadti there is a merge conflict though

[mturley]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mturley

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• clients/ui/OWNERS [mturley]

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