Backend: Add an optional description field under input
### Problem
1. Users don't know what the purpose of each input is
2. We want to display input information in the catalog by default
### Solution
Add a description field underneath inputs.
Suggested syntax:
```yaml
spec:
inputs:
environment:
default: production
description: Deployment environment to use
```
[MR](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/132040) addressing this issue.
Follow up issue - extract the description and present it in the CI/CD components catalog
issue