Add num_context_lines parameter to search API for advanced and exact code search
Follow-up from !215442
The `num_context_lines` parameter should be added to the search API with proper conditions. Everything should be in the EE version of code, since it's only available for advanced and exact code search (zoekt).
## Tasks
- [x] move field to the EE version of `search_param_keys` in `ee/lib/ee/api/helpers/search_helpers.rb`
- [x] Add optional param to `search_params_ee` in `ee/lib/ee/api/search.rb`
- [x] Update documentation in `doc/api/search.md`, noting it is only available for advanced and exact code search
## Related
- MR: !215442
- Original issue: #493281
- Revert issue: #583048