workers: event-manager: event_types: tweak event types

[akirillo]

This PR tweaks the event types to make them more ergonomic for historical state indexing. Namely, this includes:

1. Inlining the event_id and event_timestamp fields into each event type, so that it is self-contained, making it more easily matchable / handleable
2. Adding the matching_pool field to order placements & updates, to ensure that matching pool movements can be tracked
3. Adding the execution_price to matches & external matches. While the price itself can be computed from the match result, the timestamp on it is important. We could assume the event_timestamp is the timestamp of the fill, but it is important to the frontend that fills are recorded with the timestamp at which the price was sampled, specifically.
4. Removing the external tagging of the RelayerEvent. This unfortunately makes it impossible to deserialize the u128 amount field, presumable due to Untagged enum does not play nice with u128 serde-rs/serde#1682