Add HTTP connections and add support for event source tables in SQL

[mwylde]

Event sources are now configured via a connection (containing the HTTP endpoint and headers--typically auth related), which allows them to be created in SQL:

CREATE TABLE mastodon (
    value TEXT
)
WITH (
    connection = 'mastodon',
    path = '/streaming/public',
    events = 'update',
    serialization_mode = 'raw_json');