feat(combobox): implement on open event for combobox

[rathnavel]

Description

1. "combobox-on-expand" dispatch event is implemented. This event will trigger every time when we open the Combobox.
2. New property "show-loader" has been implemented. This will be helpful to show the custom loader slot content.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots:

Before (If applicable):

After:

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation update
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[parsoman]

Please resolve the conflicts