Add standard interaction patterns to GBS HTTP protocol spec

[itamarst]

The HTTP protocol spec includes a number of endpoints, but doesn't really document the interaction patterns.

For example, one can create a storage index, create a lease on a storage index, write to a share in a storage index, renew a lease... It's not immediately clear which order these operations happen in.

Some example sessions, "here's how you create an immutable", "here's how you renew it", "here's the same for mutable", "here's how you update an immutable", will make the spec clearer and probably be useful in nailing down details about things like endpoint error codes.

[exarkun]

I made a start on this at ​https://github.com/tahoe-lafs/tahoe-lafs/pull/1105

[GitHub <noreply@…>]

In 28758b81/trunk:

Merge pull request #1105 from LeastAuthority?/3764.gbs-interaction-patterns

Add some GBS interaction examples

Fixes: ticket:3764