Should not be merged before https://github.com/medusajs/medusa/pull/10895
**What**
- Introduces a new `plugin` command to `admin-bundler`, currently not used anywhere but will be called from `medusa build:plugin`
- Discovers plugins with extensions and add passes the to `admin-vite-plugin`.
- Updates `admin-vite-plugin` so its able to read built admin extensions.
Resolves CMRC-830, CMRC-839
