Asset Manager: Provider fallback

Implementers have the option to configure multiple providers for assets, for example Repo (database), S3, Azure, and others. The first one in the list has high priority and will always be used to serve assets but that provider may fail and Beacon needs to try serving the same asset from the second provider in the list, or the third one, until exhausting all the available providers.

Start by locating the Asset Manager, provider configuration, and asset-serving entry point. Trace how the prioritized provider currently serves an asset, then define the success and failure behavior across the ordered provider list. Done means the same asset is attempted from subsequent providers after failure, with behavior verified when a provider succeeds and when all providers are exhausted.

Written by the indexing model from the issue text.