Seek/Jump API for Reader/Storage

[emersonknapp]

Description

Provide a way to "seek" the "read head" of a bag to an arbitrary timestamp. This will allow for more flexible exploration and playback of bags.

Related Issues

#696 - will enable full jump functionality

Completion Criteria

• New rosbag2_storage::storage_interfaces::ReadOnlyInterface API with signature (probably) seek(timestamp) - which clamps value to duration of the bag
• Implement this API for the default sqlite3 storage implementation
• Expose this to the rosbag2_cpp::Reader and rosbag2_py.Reader so that it can be used for API-driven use case
• Make sure "seek"ing across split bag boundaries works correctly both backwards and forwards

Out of scope:

• Exposing this functionality to Player - that will be a followup issue, which will require careful management of the play queue

Implementation Notes / Suggestions

rqt_bag for ROS 2 already implements similar functionality with direct sqlite3 queries, it can provide implementation hints

Testing Notes / Suggestions

• Unit testing for new C++/Python APIs
• Test to make sure "seek"ing across split bag boundaries works correctly both backwards and forwards

[emersonknapp]

FYI @MichaelOrlov - my team will be trying to implement this within the next few weeks, which should make part of the jump function easier to do