feat: add NewFileBucket func

[jzs]

Enables the possibility to initialize FileBuckets from external packages

[jzs]

I was working on my own Bucket implementations and wanted to support both FileBucket and my own custom SftpBucket implementation when initializing a new server so i needed an exposed way of initializing FileBuckets

[bdon]

@msbarry would it be cleaner here to make Path an exported field?

[msbarry]

@msbarry would it be cleaner here to make Path an exported field?

Only if you want it to be readable. Otherwise it's kind of nice to have a function that all creation goes through in case you want to insert some logic, path normalization, etc.

[bdon]

Thanks for the PR!