• This is currently blocked by Rate limits cleanup #19990

The main change this PR introduces is to flatten the rate limit configurations.
Currently it's a dictionary containing:

• the aggregate rate limits (for messages subject to this limit)
• the aggregate byte rate limit (for messages subject to this limit)
• A dict for "transaction messages" (a name for the category of messages not subject to the aggregate limits), mapping message types to rate limits
• A dict for "other" messages, which are subject to the aggregate limits, mapping message types to rate limits
• the default rate limits for any message that doesn't have a specific limit configured (there are none)

The main changes:

• remove the "default_settings", since it's not used
• move the aggregate rate limits out into its own object
• merge the "tx" and "non-tx" lists, instead indicating whether a message type is subject to the aggregate rate limits as part of the rate limit settings.

Gains

• This removes the use of Any in the type hint, since the dictionary is now homogeneous
• The rate limit settings is a simpler structure
• There's no longer any need for a complex function to amend the v2 settings with the v1 settings. We can just use a regular dict.update()