Add a view for HTTP headers

This commit adds a migration that creates a view of the HTTP headers in the response table. Once the view is in place you can run a query like this without requiring JSON parsing:

```sql
SELECT warc_record_id, name, value FROM http_headers;
```

It can be helpful for identifying for things like:

```sql
SELECT
  value,
  COUNT(*) AS count
FROM http_header
WHERE name = 'content-type'
GROUP BY value
ORDER BY count DESC;

value                              count
---------------------------------  -----
application/javascript             57
image/png                          11
text/css                           7
text/html; charset=utf-8           6
image/jpeg                         4
image/gif                          4
text/fragment+html; charset=utf-8  3
image/svg+xml                      3
text/plain                         2
text/html; charset=UTF-8           1
```

Closes Florents-Tselai#24

README.md

@@ -52,6 +52,9 @@ Here's the relational schema of the `.warcdb` file.
 
 ![WarcDB Schema](schema.png)
 
+In addition to the core tables that map to the WARC record types there are also helper views that make it a bit easier to query data:
+
+- *http_header*: A view of HTTP headers in responses where each row is a tuple of `(warc_record_id, name, value)` 
 
 ## Motivation
 

warcdb/migrations.py

@@ -95,3 +95,16 @@ def m001_initial(db):
             ("warc_concurrent_to", "metadata", "warc_record_id"),
         ],
     )
+
+@migration()
+def m002_headers(db):
+    db.create_view(
+        "http_header",
+        """
+            SELECT
+                response.warc_record_id AS warc_record_id,
+                LOWER(JSON_EACH.VALUE ->> '$.header') AS name,
+                JSON_EACH.VALUE ->> '$.value' AS value
+            FROM response, JSON_EACH(response.http_headers)
+        """
+    )