new `key` argument for `tt cluster show` and `tt cluster publish`

[TarantoolBot]

Product: tt CLI
Since: with Tarantool 3.0
Audience/target: all
Root document: https://www.tarantool.io/en/doc/latest/reference/commands/
SME: @ psergee @ oleg-jukovec

Details

To do in scope of #3725

The patch adds a new argument key that could be specified via
etcd URI:

• key - a target configuration key in the prefix.

A user can now perform operations (show and publish) on
individual keys in etcd, not just for entire prefixes.

As example:

$ tt cluster show https://user:pass@localhost:2379/prefix?key=target

Requested by @oleg-jukovec in tarantool/tt@7fe9f11.