Move Backup extension into api module (#4392)

#### What type of PR is this?

/kind cleanup
/area core
/milestone 2.9.x

#### What this PR does / why we need it:

Move Backup extension into api module to share the extension to plugins. 

After this PR is merged, I will publish Halo to maven repository (2.9.0-SNAPSHOT). This way, developers can use the Backup extension in the plugin.

#### Does this PR introduce a user-facing change?

```release-note
None
```
pull/4394/head
John Niang 2023-08-09 10:48:11 +08:00 committed by GitHub
parent 3ec409100b
commit b9b663e124
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