Set filename for PartItems in MultipartFormDataOutput

[piu130]

Description

There is currently no way to set the filename for parts in MultipartFormDataOutput for resteasy-reactive.
Here is the corresponding code.

Implementation ideas

Either add a method

// or void
public PartItem addFormData(String key, PartItem entity)

or

public PartItem addFormData(String key, Object entity, MediaType mediaType, String filename)

[geoand]

@piu130 I think adding the first option makes sense. Would you like to contribute that?

[quarkus-bot]

/cc @FroMage (resteasy-reactive), @Sgitario (resteasy-reactive), @stuartwdouglas (resteasy-reactive)

[piu130]

@geoand Sure, I'll try it.