Backend: Support inputs for triggers
## Summary
As a part of https://gitlab.com/groups/gitlab-org/-/epics/16321+; support inputs for `trigger`
- Accept `inputs` in `trigger:include` for parent-child pipelines:
```
child-pipeline:
trigger:
include: 'child.gitlab.ci.yml'
inputs:
security_scan: true
level: 20
```
- Accept `trigger:inputs` for multi-project pipelines"
```
multi-project-pipeline:
trigger:
project: group/project
inputs:
security_scan: true
level: 20
```
**NOTES:**
- Ensure `execute_params` has `inputs` included
- Ensure that `trigger:inputs` and `trigger:include:inputs` are mutually exclusive.
issue