Implementation

1. Implement AllDatabasesChangeStreamShardTargeterImpl module according to the design at https://docs.google.com/document/d/1aZxeZ2FIh9Sn1KYotp-ac4G2AjzrT4hDKkChe4hiFpM/edit?tab=t.0#bookmark=id.qijt873rif8d.
   1. The skeleton is available at mongo/src/mongo/s/change_streams/all_databases_change_stream_shard_targeter_impl.{h,cpp}
   2. Enable v2 change streams implementation for all-databases (also called “whole-cluster” elsewhere) change streams at https://github.com/10gen/mongo/blob/b5a8c3a7e61274321b2f1715933847c33b0699f3/src/mongo/db/pipeline/document_source_change_stream.cpp#L358-L360.

Testing

1. Develop a unit test for AllDatabasesChangeStreamShardTargeterImpl.
2. Modify `implicit_change_stream_v2.js` override to support all-databases change streams.
3. Create the following new test suites for all-databases change streams with v2 change stream reader reader:
   1. change_streams_whole_cluster_mongos_passthrough_v2 (from change_streams_whole_cluster_mongos_passthrough)
   2. change_streams_whole_cluster_secondary_reads_sharded_collections_v2 (from change_streams_whole_cluster_secondary_reads_sharded_collections)
4. Develop a smoke test (performs basic functionality testing) for Ignore-Removed-Shards mode for all-databases change streams.
   1. Name it jstests/sharding/query/change_streams/change_stream_all_databases_v2_ignore_removed_shards_smoke.js
   2. Can get some inspiration from https://github.com/10gen/mongo/blob/master/jstests/sharding/query/change_streams/change_stream_database_v2_ignore_removed_shards_smoke.js.