Add "app" vfolder usage mode to compose the container filesystem

Let's make our commonly used kernel runner binaries and applications (such as ttyd, vscode, etc.) "composable", so that we can merge multiple of them and inject the directories/files into the compute containers.

There may be multiple ways to implement a catalog of such "container libraries", but a special type of vfolder should do a good job because:

- Updating and redistributing an app library is as simple as just updating the app vfolder.
- Testing and packaging becomes also as simple as just compressing required files.
Things to implement:

- a new vfolder usage mode: `APP` (in addition to existing `GENERAL`, `MODEL`, `DATA`)\* When creating a new container, the agent should take care of extending the container creation arguments such as mount configurations and/or environment variables depending on the usage mode.
- While #665 involves **vfolder backends** to specify how the contents of vfolders are provided/sourced, **the vfolder usage mode** should handle actual population of the files and directories inside the container.\* Q: Could we use any low-level overlayfs APIs instead of specifying a (potentially long) list of individual mount options of the Docker/CRI container creation API?
- We need to define the role and responsibility of vfolder backend and vfolder usage mode more clearly.

- a fast/cached vfolder metadata query API\* We need to define `metadata.json` schema to cover app vfolders as well.
- https://github.com/lablup/backend.ai/blob/f43b7966e9c14f83fba203f532bc9d93e504d900/src/ai/backend/storage/vfs/**init**.py#L147\* Currently the put/get APIs for vfolder metadata are only defined in the storage proxy and there is no related APIs exposed in the manager API.
- Q: Would we need to cache a copy of `metadata.json` in the `vfolders` table?

Things to consider in the app vfolder metadata schema:

- backend-plugin compatibility check (e.g., docker, k8s, ...)
- image-plugin compatibility check (e.g., only applicable to images with specific tag)
- declaration of additional service ports & injection of service definition configs by plugins\* When creating new containers, the image label `ai.backend.service-ports` should be merged with the plugin-declared service ports and set as the container label.

- generic mount lists based on package resource paths\* Depending on the app, it may be a single large directory that contains every dependency or a list of multiple directories and files.

```Java
### Tasks
- [ ] #1066
```

JIRA Issue: BA-273