Support variant fileSchema values

[jwodder]

s3invsync currently requires the fileSchema fields in manifest.json files to equal:

"Bucket, Key, VersionId, IsLatest, IsDeleteMarker, Size, LastModifiedDate, ETag, IsMultipartUploaded"

Relax this requirement.

@yarikoptic Do you want/need this for the first working version? It'd be rather tedious to implement.

[yarikoptic]

I am missing why is it tedious, must be a rust thing since sounds just like loading/constructing a dict and e.g. mapping into dataclass attributes or just using as a dict throughout. But sure -- it is not a priority, but I bet would become needed for anyone to adopt the tool for their bucket inventory with their custom setup.