CDRIVER-4363 Support multiple OP_MSG doc sequences (#1578)

Support sending an OP_MSG with two document sequences (OP_MSG Section with payloadType=1). Intended for upcoming support of the `bulkWrite` command. Drivers are expected to send document sequences for both `nsInfo` and `ops` fields.

* remove unnecessary `_mongoc_get_documents_field_name`

The payloadType=1 identifier is already available in `mongoc_cmd_t::payload_identifier`

* define a `mongoc_cmd_payload_t` struct

* define `MONGOC_CMD_PAYLOADS_COUNT_MAX`

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Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>