ICS-2: `ClientType` is not defined and confusing

[plafer]

The type ClientType is not clearly defined in ICS-2, and seems to play 2 different roles.

On the one hand, it is the type of the value stored at "clients/{identifier}/clientType" in the provableStore (see ICS-24 table). Given how it's meant to be used, I would expect it to be a well-defined type (as opposed to an opaque data structure). For example, in ibc-go, ClientType is a string.

On the other hand, the reader can gather that it is an object that is expected to have the methods such as initialise(), checkValidityAndUpdateState(), and checkMisbehaviourAndUpdateState(). I was able to figure that out by looking at how it is used in the code snippets, and then link it to the text that says

Client types MUST define a method to initialise a client state with a provided consensus state, writing to internal state as appropriate. [the initialise method]

It wasn't immediately clear to me that the quote above referred to the ClientType type.

Given this, what is ClientType supposed to be: the former or the latter?

Finally, we should also ensure that the ClientType value "07-tendermint" that ibc-go uses to identify the Tendermint client is documented somewhere.