Tezlink: add describe endpoint to tezlink directory

What

Adds a /describe endpoint to the /tezlink directory.

Why

We plan to use url-of-the-l2-node:port/tezlink as the endpoint of the tezos compatible node, however tezos tools (e.g. octez-client) expect the endpoint to offer /describe. We have already added a /describe endpoint to the octez-evm-node but it is in the root directory, not /tezlink.

How

Just add the endpoint when building the directory.

Manually testing the MR

The existing test of the /describe (test_describe_endpoint) has been extended to also target /tezlink/describe/ and is done with a sequencer running a tezlink instance, so the output is extended with all tezlink specific rpc.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR