CDRIVER-5925 apply batchSize:0 to aggregate in change stream

[kevinAlbs]

Summary

Apply batchSize:0 to aggregate for change streams.

Tested with this patch build.

Background & Motivation

WRITING-30021 reported an outage due to runaway aggregate operations overwhelming the server.

This PR supports setting batchSize:0 on the aggregate command sent to create a change stream cursor. This is intended to help the driver kill the server-side cursor when destroying the client handle.

Sending batchSize:0 can help cancel runaway aggregate commands on the server. batchSize:0 results in aggregate immediately returning with a cursor ID and no results. Knowing the cursor ID enables mongoc_change_stream_destroy to send the killCursors command to kill the server-side cursor.

Using batchSize:0 for aggregate is supported as a way to create a server-side cursor immediately. Quoting HELP-29972:

Both the aggregate command and find command support establishing a cursor with batchSize:0 exactly for this reason

WRITING-10467 proposes a long-term solution for all drivers: a new option emptyInitialBatch. The new option would also permit a non-default batchSize for getMore. A non-default batchSize for getMore is not needed to address the reported issue. This PR is intended to be a simpler short-term solution to address the urgent issue while avoiding a conflict with the spec.

Existing behavior with batchSize:0

The C driver currently ignores batchSize:0 option when creating a change stream. This results in using the server default for both the aggregate and getMore commands. Behavior for batchSize:0 differs between drivers.

In the C/C++ driver (full example):

mongocxx::options::change_stream options;
options.batch_size(0);
mongocxx::change_stream stream = collection.watch(options); // Does not send batchSize for aggregate or getMore.

In PyMongo (full example):

with collection.watch(batch_size=0) as stream: # Sends batchSize:0 for aggregate. Does not send batchSize for getMore.

This PR changes the C driver to behave as PyMongo.

Caveats

This is a subtle behavior change. Specifying batchSize:0 for change streams in the C driver is currently ignored. If a user were (unexpectedly?) setting batchSize:0, this may result in an additional needed round-trip to get initial results (the first aggregate would now return empty results). I expect this is low risk to cause negative impact.

[ghost]

Looks good to me!