Add support for more span queries (#553)
Add support for more span queries (#553)

Add support for the `span_containing`, `field_masking_span`, and
`span_within` span queries.

Signed-off-by: mattjmcnaughton <d0388760b91d86bb40e560651d9bf64323d45b23@gmail.com>
elastic/elasticsearch-dsl-py,3lnc/elasticsearch-dsl-py,f-santos/elasticsearch-dsl-py