Add ResolveOption.AllowStale to Resolver

[bartelink]

Resolves originally used the Target alone to signal load hints. One of these being Target.AggregateIdEmpty, which signals that the system should not attempt to load the stream when necessitated by a Query or Transact, but (e.g. if the client has just minted a fresh stream id) should instead work on the assumption that no events will be loaded.

This concept generalizes to having a read mode option, which allows one to specify the following:

• AssumeEmpty - as in OP , check cache, but if no value held, assume empty stream
• AllowStale - hint that if a cached value is present that a a read should optimistically use that state and position for a first attempt (for an execution cycle, that means leaning on the conflict detection to trigger an automatic resync iff we're on a node whose cache happens to be stale wrt the given stream - for a read, it means you're not going to be able to guarantee that you're yielding the freshest state available in the cluster)
• 🤔 if Equinox.Cosmos ever needs an equivalent of Equinox.EventStore.CachingStrategy.SlidingWindowPrefixed, a prefix could potentially be supplied via this mechanism