feat: Add new App Installation Parameter value type 'Secret' [EXT-4836]

[jjolton-contentful]

Summary

When creating App Installation Parameter schemas by modifying an App Definition, users can now specify that a parameter is of type "Secret". At the time of this PR this functionality is currently only supported on the schema side.

Description

Motivation and Context

Apps will soon be able to use secret app installation parameters for storing params whose exposure should be limited.

Checklist (check all before merging)

• [x] Both unit and integration tests are passing
• [x] There are no breaking changes
• [ ] Changes are reflected in the documentation

Documentation updates to come as this feature matures.

[contentful-automation[bot]]

:tada: This PR is included in version 11.26.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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