Allow scenario state to be scoped to arbitrary request attributes

[tomakehurst]

Proposal

Currently scenario state applies globally to the mock API, meaning that one user/thread can affect outcomes for another.

To avoid this we should add a context attribute to the scenario that can be expressed as one or more request attributes (via templating would probably make most sense). This way e.g. a session cookie or JWT header value could be used to segregate user sessions.

This context value could be a single templated string. However, it may make sense for it to be possible to supply a list of templated strings so that context elements can be separated (rather than each user coming up with their own string building strategy). TBD.

References

No response