Implement @substrate-connect/discovery package

[ryanleecode]

Create a package (temporarily named "substrate-extension-discovery") that enables the discovery of Substrate extensions. This package should expose a single discover() method with the following type signature: async fn discover() -> Extension[].

The Extension type should have the following properties:

• providerName: 'Substrate Connect'
• kind: 'smoldot' | 'unstable-substrate-connect'
• payload: { addChain: ..., addWellKnownChain: ... }

Note: The payload for 'unstable-substrate-connect' kind should match the interface we want to expose for the Substrate Connect extension.

When a user installs this library and runs the discover() method, it should return an array of extensions implementing the desired interface (i.e., either 'smoldot' or 'unstable-substrate-connect' kind).

The user should then be able to interact with the extensions using the provided interfaces.

Implementation Notes:

• Define the expected interface type in this new package.
• Ensure our extensions return the expected interface type.

This approach allows us to define the interface without moving existing code.

[ryanleecode]

#2200