feat: local backend

[flavio]

Note: this PR builds on top of https://github.com/FiloSottile/sunlight/pull/19

I really love how easy it is to sunlight for testing/playing purposes. I've noticed however that sunlight requires access to a S3-compatible server to store the logs data. I've added the possibility to the local filesystem instead of relying on a S3 bucket. This is not meant to be used in production, it just simplifies the setup for testing purposes.

I hope you like this change. Thanks again for the great tool!

Just for reference, this is the configuration file I used to spin up a local testing instance:

listen: ":8080"

checkpoints: checkpoints.db

logs:
  - name: sunlight.127.0.0.1.sslip.io/2024h1
    shortname: bergamo2024h1
    inception: 2024-06-28
    httpprefix: /2024h1
    roots: ./chain.pem
    key: ./key.pem
    cache: bergamo2024h1.db
    poolsize: 750
    notafterstart: 2024-06-26T00:00:00Z
    notafterlimit: 2024-07-01T00:00:00Z
    localbackend: local_backend