ReadFileAsync: Only int lengths/offsets are currently supported

[FileSystemApi.ReadFileAsync](https://github.com/ipfs-shipyard/net-ipfs-http-client/blob/main/src/CoreApi/FileSystemApi.cs#L194) does not allow Int64 for offset and length. 

This limitation can be removed, Kubo [/cat](https://docs.ipfs.tech/reference/kubo/rpc/#api-v0-cat) now supports [int64] in these fields.
