add stub API for client bulk write

[kevinAlbs]

Summary

This is a stub to implement the client bulk write API proposed in mongodb/specifications#1534.

This is intended for early feedback, but not to merge.

Rationale

Options

The existing collection bulk write API accepts options in bson_t *opts:

MONGOC_EXPORT (mongoc_bulk_operation_t *)
mongoc_collection_create_bulk_operation_with_opts (
   mongoc_collection_t *collection,
   const bson_t *opts) BSON_GNUC_WARN_UNUSED_RESULT;

The proposed client bulk write API differs. Setter functions are added for improved type safety and editor auto complete:

BSON_EXPORT (void)
mongoc_bulkwriteoptions_set_ordered (mongoc_bulkwriteoptions_t *self, bool ordered);
// ... and other `mongoc_bulkwriteoptions_set_*` setter functions ...

Return

The existing collection bulk write API reports results and error information in a bson_t *reply:

MONGOC_EXPORT (uint32_t)
mongoc_bulk_operation_execute (mongoc_bulk_operation_t *bulk, bson_t *reply, bson_error_t *error);

This PR notably differs. Getter functions are added for improved type safety and editor auto complete:

BSON_EXPORT (int64_t)
mongoc_bulkwriteresult_insertedcount (const mongoc_bulkwriteresult_t *self);
// ... and other `mongoc_bulkwriteresult_*` getter functions ...

Map types are still reported in a bson_t:

// `mongoc_bulkwriteresult_verboseresults` returns a document with the fields: `insertResults`, `updateResult`,
// `deleteResults`. Returns NULL if verbose results were not requested.
BSON_EXPORT (const bson_t *)
mongoc_bulkwriteresult_verboseresults (const mongoc_bulkwriteresult_t *self);

There is no existing convenient representation of map types. I expect needing to interact with verbose results is rare.

Errors are returned in mongoc_bulkwriteexception_t:

// `mongoc_bulkwriteexception_error` sets `error_document` to a document with the fields: `errorLabels`,
// `writeConcernErrors`, `writeErrors`, `errorReplies`.
BSON_EXPORT (void)
mongoc_bulkwriteexception_error (const mongoc_bulkwriteexception_t *self,
                                 bson_error_t *error,
                                 const bson_t **error_document /* May be NULL */);

The maps and lists are reported in the bson_t **error_document.

The mongoc_bulkwriteexception_t can outlive the mongoc_bulkwrite_t. This is intended to enable wrapping languages to wrap mongoc_bulkwriteexception_t when throwing an exception.

[kevinAlbs]

Closing. #1590 proposes an API intended for merging.