Add an in-memory and default implementation of detached blob storage

[anthony-murphy]

Based on feedback from partners, the current story around blobs is overly complicated, and hard to work with. The existing detached blob storage interface is difficult to work with as it applies to the whole loader, and not a specific container, additionally it is decoupled from the serialization flow which make coordination difficult, lastly the need to supply detached blob storage when not using serialization is burdensome.

To address these issues, this change creates a default in memory detached blob storage which also support serialization and deserialization, so that detached, serialization and rehydration can be used in the presence of blobs without the need to pass a custom storage mechanism which doesn't align with our existing surface area.

For testing I've refactored our existing detached blob storage to support testing with both a custom detached blob storage specified, and an undefined detached blob storage which will result in using the new in memory storage. These test cover attaching, serializing, and deserializing with blobs.