ingest: support custom_rules from config

Follows the pathogen-repo-guide to allow users to define custom rules
in the config to extend or override the core ingest workflow.

This is needed to support the upcoming ingest tutorials which use
this repo as the example.

ingest/Snakefile

@@ -71,3 +71,9 @@ if send_slack_notifications:
 if config.get("trigger_rebuild", False):
 
     include: "rules/trigger_rebuild.smk"
+
+
+if "custom_rules" in config:
+    for rule_file in config["custom_rules"]:
+
+        include: rule_file