Add `parse_strings_from_args` to get `prev_events` array (#10048)

Split out from https://github.com/matrix-org/synapse/pull/9247

Strings:

 - `parse_string`
 - `parse_string_from_args`
 - `parse_strings_from_args`

For comparison with ints:

 - `parse_integer`
 - `parse_integer_from_args`

Previous discussions:

 - https://github.com/matrix-org/synapse/pull/9247#discussion_r573195687
 - https://github.com/matrix-org/synapse/pull/9247#discussion_r574214156
 - https://github.com/matrix-org/synapse/pull/9247#discussion_r573264791



Signed-off-by: Eric Eastwood <erice@element.io>