Add new geo alias for gismit and gisogm sources

Why these changes are being introduced:

With the addition of new sources 'gismit' and 'gisogm', it has been decided these sources will also funnel into
a new alias called 'geo'.  This alias will provide a geo-only index for powering a geo focused UI.

How this addresses that need:
* Adds new 'geo' alias and sources that map to this alias

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/GDT-92

lambdas/config.py

@@ -4,6 +4,7 @@
 INDEX_ALIASES = {
     "rdi": ["jpal", "whoas", "zenodo"],
     "timdex": ["alma", "aspace", "dspace"],
+    "geo": ["gismit", "gisogm"],
 }
 REQUIRED_ENV = {
     "TIMDEX_ALMA_EXPORT_BUCKET_ID",