Synchronise Lookup institutions.

This causes Lookup institutions and their members to be synchronised with groups in Google, in a similar way to Lookup groups. The unique identifier of the group in Google is it's email address:

email = {instID.lower()}@{insts_domain}

where {insts_domain} is a configurable secondary domain (noting that the local part of the email address is required to be lowercase, whereas all instIDs from Lookup are uppercase).

Closes #7 (closed)

gsuitesync/gapidomain.py

@@ -25,3 +25,7 @@ class Configuration(ConfigurationDataclassMixin):
     # Secondary domain or domain alias for groups. If None, the value of "name" is used.
     # Default: None
     groups_domain: typing.Union[str, None] = None
+
+    # Secondary domain or domain alias for institutions. If None, the value of "name" is used.
+    # Default: None
+    insts_domain: typing.Union[str, None] = None