Filtered Observers

## Goal Description

When working with observers, it is common to want them to only listen for events that target entities which match a certain criteria. The existing solution requires generating a query, and then early-returning if the targeted entity does not match. This should have less overhead and be more ergonomic. When completing this work, we may also want to consider removing the `B: Bundle` generic for observers to reduce confusion.

Previously: #15287. Synergistic with #23018, but neither blocks the other.

## Goal Status

This is a potential [Bevy Project Goal](https://bevy.org/learn/contribute/project-information/project-goals/). Its state on the [Project Goals Board](https://github.com/orgs/bevyengine/projects/23/views/1) determines if it is Proposed, Postponed, Blocked (Approved), Inactive (Approved), Active (Approved), Completed, or Declined.

- **Staffing SMEs**: Not yet staffed
- **Working Group Link**: Working Group not yet formed
- **Design Documents**: Design documents not yet written

Start by reading this issue alongside the previously related #15287 and synergistic #23018, then check the Project Goals Board for its current status. No implementation files, tests, design documents, or staffed working group are identified, so the completion criteria remain to be defined before coding can begin.

Written by the indexing model from the issue text.