Add eqxtestbed template

[bartelink]

This is extracted from what eqx run does at the indirect request of @jakzale - it provides a test harness that will enable one to write/tweak a prototype of a particular way of modelling a stream and it's transactions, and then validate that by running variable numbers of synthetic clients over a definited period of time, gathering and presenting latency and RU metrics at the end of a run.

• Initial impl with support for EventStore, Cosmos, MemoryStore
• make a subcommand mandatory (i.e. don't default to memory store, to avoid confusion)
• lower default iterations to something that will definitely work interactive against QA (100?)
• Mention in Top-level Readme; perhaps add one here?
• Remove clone of Equinox.Tools.TestHarness; switch to referencing the nuget version of that
• Make es and cosmos switchable in the template (will default to both on) @bartelink
• Polish testbed, tweak logging in Equinox.Tools.TestHarness @jakzale
• Move any tweaks in this Equinox.Tools.TestHarness back into Equinox (all logic in that library is pretty much unmodified from the original by @eiriktsarpalis) @jakzale