Update API models to use envelope format consistently

[alexcreasy]

Description

Updates the BFF API contract to use envelopes more consistently.

• All endpoints now use the standard data key for the primary payload
• POST/PATCH endpoints now accept a request body that matches the envelope format and formats the response body in the same way

How Has This Been Tested?

• Run through the examples CURLs in the README
• Verify that the endpoints use the new envelope format { "data": ... } for both request and response bodies

Merge criteria:

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

• The commits have meaningful messages; the author will squash them after approval or in case of manual merges will ask to merge with squash.
• 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.

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.

[ederign]

.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ederign

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• clients/ui/OWNERS [ederign]

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[ederign]

/lgtm