BE: Closes #71 Messages: Show headers duplicates

[PopClom]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)
Closes #71
Added support for duplicate header keys. If a header holds a single value, it is represented as a string (as it currently works). If it holds multiple values, it's represented as a list of strings. Also added a quick example in the smart filter info modal.

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[Haarolean]

this doesn't look good. I believe in case we have one header value we still have to have a list but of one single element.

[PopClom]

I implemented it this way to ensure headers can still be passed in the same format as before. Otherwise, users would always need to provide a list, which could break compatibility with existing inputs.

[Haarolean]

backward compatibility issues are less evil here