Asset Hub: auto incremented asset id for trust backed assets (#5687)

Setup auto incremented asset id to `50_000_000` for trust backed assets.

In order to align with Polkadot/Kusama Asset Hub -
https://github.com/polkadot-fellows/runtimes/pull/414
The next closes existing assets IDs in Rococo is `69_696_969`, in
Westend is `88_228_866`.

### Migration
**Stakeholders**: all clients providing asset creation functionality on
Westend/Rococo Asset Hub

This change does not break the API but introduces a new constraint. It
implements an auto-incremented ID strategy for Trust-Backed Assets (50
pallet instance indexes on both networks), starting at ID 50,000,000.
Each new asset must be created with an ID that is one greater than the
last asset created. The next ID can be fetched from the `NextAssetId`
storage item of the assets pallet. An empty `NextAssetId` storage item
indicates no constraint on the next asset ID and can serve as a feature
flag for this release.