Tezt: add a suffix to the endpoint representation

What

Adds the possibility to represent endpoints of the form http://0.0.0.0:1234/suffix

Why

We want to use the octez-client to communicate with the l2 node we're building for Tezlink. That node is built upon the octez-evm-node. For technical reasons we need to prefix all paths: the root is /tezlink/ and not just /.

How

Add a path field to Endpoint.t.

To avoid disrupting the usual use of Endpoint.t, add a smart constructor for Endpoint.t with an optionnal argument for path.

Manually testing the MR

It's a refactoring, we will be using the new feature in a following MR: 3c35dfa4