Allow `bevy_asset` to operate in many environments.

## Goal Description

Assets are used throughout Bevy, making it a requirement for most users. However, the current asset system is tightly coupled to our current asset loading implementation. Assets should be capable of being used in as many environments as possible, in as many ways as possible. This involves several cases:

1. `no_std` platforms: the asset system should be able to operate on ~every platform.
2. Alternate loading strategy: users should be able to completely avoid the asset server and create "their own" asset server.

## 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**: @andriyDev
- **Working Group Link**: Working Group not yet formed
- **Design Documents**: Design documents not yet written

Start with the bevy_asset subsystem and review this goal's status on the Bevy Project Goals Board. No design documents or working group exist yet, so clarify the scope for no_std platforms and alternate loading strategies with the listed staffing SME before identifying implementation entry points. Done requires an agreed design and project plan.

Written by the indexing model from the issue text.