Passing additional CLI arguments to intrinsic container apps

Currently there is no way to pass additional CLI arguments to intrinsic container apps such as ttyd and dropbear, in contrast to [image-based apps with service definitions](https://docs.backend.ai/en/latest/dev/adding-kernels.html#service-definition-dsl).

Let's allow this to:

- Customize fonts of ttyd\* refs https://github.com/tsl0922/ttyd/issues/31#issuecomment-432901658
- [The current implementation](https://github.com/lablup/backend.ai-agent/blob/main/src/ai/backend/kernel/intrinsic.py#L127) does not provide means to pass custom args in the right place.

- Support VSCode as intrinsic app (#59)
Also related with:

- #229
- #399

JIRA Issue: BA-272