Give StaticFileConfig a proper toString

[MariusVolkhart]

This gets invoked and presented on the console during startup, and shows which mimetypes are registered for a file extension. Previously, the message included something like

2024-01-09 21:20:03,840 INFO  i.j.Javalin Static file handler added: StaticFileConfig(
hostedPath=/, directory=src/main/resources/public, location=EXTERNAL, precompress=false, 
aliasCheck=null, headers={Cache-Control=max-age=0}, 
mimeTypes=io.javalin.http.staticfiles.MimeTypesConfig@46d567cb). 
File system location: 'src/main/resources/public'

Note the mimeTypes=io.javalin.http.staticfiles.MimeTypesConfig@46d567cb.

It now is formatted as mimeTypes={csv=text/csv}, or just {} if it's empty.

[tipsy]

Thanks @MariusVolkhart !