add example of collection with a custom handler

collections/sample_custom_processor.yaml

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+Name: test_custom_processor
+Title: test_custom_processor
+EodashIdentifier: test_custom_processor
+Description: ""
+Themes:
+  - example theme
+Tags:
+  - example tag
+DataSource:
+  Spaceborne:
+    Satellite:
+      - example satellite
+    Sensor:
+      - example sensorname
+Agency:
+  - EEA
+Resources:
+  - Name: Custom-Endpoint
+    Python_Function_Location: "custom_handlers.custom_endpoint.execute"
+    CustomParameter: "test"
+    STAC_Url: "https://s3.us-west-2.amazonaws.com/umbra-open-data-catalog/stac/catalog.json"
+    Subset_Dates: 
+      - "2023-01-16"
+      - "2023-04-16"

custom_handlers/custom_endpoint.py

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+from pystac import Collection, Catalog
+from pystac_client import Client
+
+
+def execute(
+    collection: Collection,
+    catalog_config: dict,
+    endpoint_config: dict,
+    collection_config: dict,
+):
+    if "cerulean" not in catalog_config["title"].lower():
+        raise Exception("This demo handler should be run only on cerulean catalog.")
+    stac_endpoint_url = endpoint_config["STAC_Url"]
+    api = Client.open(stac_endpoint_url)
+
+    # catalog structure is {year}/{year-month}/{year-month-day}/items
+    for date_entry in endpoint_config["Subset_Dates"]:
+        year, month, day = date_entry.split("-")
+        catalog: Catalog = (
+            api.get_child(year)
+            .get_child(f"{year}-{month}")  # type: ignore
+            .get_child(f"{year}-{month}-{day}")  # type: ignore
+        )
+
+        for item in catalog.get_items():
+            collection.add_item(item)
+    return collection