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

Look at the input parsing logic in the codebase, likely in a config or schema package. Understand how CEL (Common Expression Language) expressions are currently evaluated, if at all. The change involves adding a 'skip_if' field to input definitions and modifying validation to conditionally ignore inputs. Check existing tests for input handling to see how to add new functionality.

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