Proposal: gitea dump should try to detect type from filename, if possible

[DanielGibson]

Feature Description

If a filename is given for gitea dump (-f or --file) and it has a file ending that corresponds to a supported --type and --type is not set, the type that corresponds to the filename should be automatically used.

For example, gitea dump -f /path/to/mydump.tar should create a tar (currently it would create /path/to/mydump.zip)

Not strictly necessary bonus-feature: Support some common alternative file-endings (like tgz for .tar.gz or .tbz for .tar.bz2)

[wxiaoguang]

Done by #30240