Swift 6 Support

[claucambra]

There is some work to make some structs sendable (e.g. the work that led to creating a SendableItemMetadata) but there is still a bunch of work that has to be done both within NCFPK as well as NextcloudKit to avoid warnings and errors when compiling in strict Swift 6 mode

[i2h3]

See branch feat/swift-6.1 for current progress.