s3:pylibsmb: Add .savefile() API to SMB py bindings

s3:pylibsmb: Add .savefile() API to SMB py bindings

This provides a simple API for writing a file's contents and makes the
s3 API consistent with the s4 API.

All the async APIs here support SMBv2 so we don't need to use the sync
APIs at all.

Note that we have the choice here of using either cli_write_send() or
cli_push_send(). I chose the latter, because that's what smbclient uses.
It also appears to handle writing a large file better (i.e. one that
exceeds the max write size of the underlying connection).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>