Publish Topic Panel Improvements

[maxwellmlin]

Not priority for Spring 2024, but would be nice to have in a future year.

• When user changes schema name, can we add an empty request object with that schema, as is provided by Foxglove's built-in Publish panel?

• Before publishing, verify that the message conforms to the schema selected. This means all required fields are included, and all values have the expected types. For example, I should get an error when I try to publish { "data": "test" } with schema std_msgs/Float32.

• When I publish on a topic with a given schema, then change the schema and publish to the same topic, a publisher with the new schema cannot be created for the same topic, due to ROS limitations. An error is displayed in the Foxglove "Problems" list. Can we also display an error in the panel itself?