Add ability to configure inputs when triggering MR pipelines
### Problem
Users can trigger merge request pipelines from the MR pipeline tab, but only with default input values. Unlike branch pipelines which can be triggered with custom inputs (and variables) via the "New pipeline" page.
Users would like the ability to customize inputs value for MR pipelines
### Proposed Solution
Enhance the merge request pipeline tab to support input configuration, similar to how branch pipelines can be triggered with custom inputs from the main pipelines page.
### MR widget:
Has only an option to run pipeline without customizing inputs
{width=1036 height=183}
### New pipeline:
Has an option to customize inputs (and variables), but its only applies to branch pipelines
{width=1608 height=607}
### MVC proposal
We can start with API only as an MVC and work on the FE part in future iteration
issue