#968 Feat(addon): addon applications, and DefKit modules (experimental) (16/21)

Stack 16/21 · after #967 · next #969

Targets main but builds on #967, so the earlier stack commits show here too: review the last commit only. Merge after #967.

Description of your changes

• Addon applications: Applications an addon installs are listed on the Addons page, not among services, and are read-only.
• DefKit (experimental): definitions written in Go with DefKit, installed from a repository as an Application. A review shows what would change and which conflicts to take over before anything is applied.

Fixes #

I have:

• Read and followed KubeVela's contribution process.
• Related Docs updated properly. In a new feature or configuration option, an update to the documentation is necessary.
• Run yarn lint to ensure the frontend changes are ready for review.
• Run make reviewableto ensure the server changes are ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

Special notes for your reviewer

• DefKit needs the defkit addon, which renders modules in a Job; that addon goes to the catalog separately. Without it the page says the addon is needed and Add is disabled.