TLA+ specification for client consistency

[heidihoward]

This PR adds a new TLA+ specification for client consistency guarantees

Following on from the discussion in #5636

[achamayou]

@heidihoward this potentially very interesting to users building applications and APIs, and so ought to be featured in the documentation in my opinion.

There is a TLA+ Specification under Architecture in the documentation (which probably needs a refresh, from having a quick glance at it). But I wonder if it wouldn't best to start a new top-level "Specifications", with "Consensus" and "Client Consistency" sub-entries.

What do you think?

[heidihoward]

@heidihoward this potentially very interesting to users building applications and APIs, and so ought to be featured in the documentation in my opinion.

There is a TLA+ Specification under Architecture in the documentation (which probably needs a refresh, from having a quick glance at it). But I wonder if it wouldn't best to start a new top-level "Specifications", with "Consensus" and "Client Consistency" sub-entries.

What do you think?

Sounds good, will save it for another PR though to get this merged