Support reading/writing chunks in remote fs

[bpasero]

This is a follow up from #32503 where IFileService.updateContent() now accepts a ITextSnapshot to prevent loading the entire buffer into memory. I left a TODO here to use ITextSnapshot directly and not the current fallback snapshotToString

[jrieken]

Yeah, goes along with these todos in the write function (see update below)

		// todo@joh - have an option to create iff not exist
		// todo@remote
		// offset - byte offset to start
		// count - number of bytes to write
		// Thenable<number> - number of bytes actually written
		write(resource: Uri, content: Uint8Array): Thenable<void>;

[jrieken]

Plan is to have more explicit, posix-style, calls that open/close a file and that read arbitrary content of a file. Something like this:

open(uri: Uri, opts): number | Thenable<number>;
close(fd: number): void | Thenable<void>;
read(fd: number, pos: number, into: Uint8Array): number | Thenable<number>;
write(fd: number, pos: number, data: Uint8Array): number | Thenable<number>;

This will compliment the simple readFile and writeFile methods and are optional (tho recommended) and it also means the implementing side needs to keep some state around.