Inputs that are conditionally skipped

### TL;DR

Sometimes, one input is unnecessary, depending on the value of another input. For example, you might have two inputs `use_postgres`, and `postgres_config_file`; if `use_postgres` is false then you don't care about `postgres_config_file`.

One way this could look: we could add a CEL expression `skip_if` to each input. For example:

```
inputs:
- name: use_postgres
- name: postgres_config_file
default: 'postgres.conf'
skip_if: '!bool(use_postgres)'
```

cc @gjonathanhong