feat(CosmosStore): Expose ISyncContext.SessionToken

[bartelink]

This PR exposes a SessionToken on Equinox.Core.ISyncState, which enables logic in a Service to access that (along with the Version plumbed in #194) via the QueryEx and TransactAsyncEx APIs. More importantly, it enables one to flow the SessionToken out to the caller in order to enable Read-Your-Writes guarantees for a system's overall client session.

This could be merged in the 2.0 timeframe, but leaving it in the 3.0 milestone for now as doing so only makes sense if Equinox.Cosmos (or another store) actually wires through the SessionToken correctly, which involves addressing #192:

• when creating the Equinox.Stream, one needs to be able to supply the incoming SessionToken (maybe add a sessionToken argument in Resolve?
• when running a Load internally, the suplied token needs to be passed in the Options
• the stored procedure and/or the invocation logic needs to surface the SessionToken at the conclusion of the Sync operation
• the SessionToken needs to flow back into the StreamToken that's yielded back

cc @thednaz @ylibrach