initial work to add azure token

This currently doesn't work to get index files, because Herbie was appending urls with the index suffix. I'll need to rethink this.

herbie/azure_token.py

@@ -0,0 +1,67 @@
+import requests
+from datetime import datetime, timezone
+
+
+class AzureBlobToken:
+    """Get a token for accessing Azure Blob Storage.
+
+    This class fetches a token from the Planetary Computer SAS API.
+    https://planetarycomputer.microsoft.com/docs/concepts/sas/
+
+    Parameters
+    ----------
+    collection : str
+        Name of the collection to get a token for.
+
+    Attributes
+    ----------
+    collection : str
+        Name of the collection.
+    url : str
+        URL to fetch the token from.
+    token_data : dict
+        Token data returned from the API.
+    token : str
+        The token string.
+    expiry_time : datetime
+        The expiry time of the token.
+    """
+
+    def __init__(self, collection):
+        self.collection = collection
+        self.url = (
+            f"https://planetarycomputer.microsoft.com/api/sas/v1/token/{collection}"
+        )
+        self.token_data = None
+        self._fetch_token()
+
+    def _fetch_token(self):
+        """Fetch the token data from the URL and set attributes."""
+        response = requests.get(self.url)
+        if response.status_code == 200:
+            self.token_data = response.json()
+            for key, value in self.token_data.items():
+                setattr(self, key.replace(":", "_"), value)
+            self.expiry_time = datetime.strptime(
+                self.msft_expiry, "%Y-%m-%dT%H:%M:%SZ"
+            ).replace(tzinfo=timezone.utc)
+        else:
+            raise Exception(
+                f"Failed to fetch token: {response.status_code} {response.text}"
+            )
+
+    def _is_expired(self):
+        """Check if the token has expired."""
+        return datetime.now(timezone.utc) >= self.expiry_time
+
+    def __str__(self):
+        """Return the token string, refreshing it if expired."""
+        if self._is_expired():
+            self._fetch_token()
+        return self.token
+
+    def __add__(self, other):
+        return f"{other}?{self.token}"
+
+    def __radd__(self, other):
+        return f"{other}?{self.token}"