A0-3509: Purge chain-extension: frontend (#1498)

# Description

Do you remember our shady and dirty chain extension? Well, we are back.

Since OBCE is no longer maintained, we have to rework the extension from
the very ground. To avoid huge diff, I divided the work into couple of
separate changes. This PR:
 - removes all OBCE references
 - brings back plain frontend part (i.e. the API for the contracts)
 - creates an example contract for testing purposes


## Type of change

Please delete options that are not relevant.

- Bug fix (non-breaking change which fixes an issue)

# Checklist:

- I have added tests
- I have made neccessary updates to the Infrastructure
- I have made corresponding changes to the existing documentation
- I have created new documentation

.github/workflows/_unit-tests-and-static-checks.yml

@@ -46,3 +46,6 @@ jobs:
         with:
           command: test
           args: -- --skip clique_network
+
+      - name: Run tests for baby liminal chain extension
+        run: cd baby-liminal-extension && make test-frontend