Update vendors overview

[lebe1]

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Description

IMPORTANT TO KNOW

CHANGES

• Update swagger and keycloak requests for vendor overview

TODO

• Add open payments to response

Checklist:

• [ ] I have commented my code (or ChatGPT did), particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings, neither in my IDE nor in my browser
• [ ] I have added tests that prove my fix is effective or that my feature works

[lebe1]

@nanu-c do you mind adding open payments of the vendor into the response and maybe one test into handlers_test.go? I think after that we can merge this PR.

[nanu-c]

I would propose to add more functionality in the following pr.

[lebe1]

Okay I like that way of workflow. I have created a new issue #124 for it. Therefore, you can approve and merge it :)