refactor: prepare the static CT client for a second tile format

[mimi89999]

First step of #105 (comment)

[mimi89999]

@d-Rickyy-b could you please review this pull request?

[messede-degod]

+1, i'm excited about this as well

[d-Rickyy-b]

Sorry for the long wait again, life is happening. The changes look good to me, I just have some minor complaints that need to get addressed.

[d-Rickyy-b]

I know this PR is made in preparation for implementing "names tiles" support (#105). Allow me one additional general comment tho to prevent confusion further down the road.

certstream-server-go is not only used to obtain the actual certificates but also to collect metadata about the CAs - e.g. via the grafana dashboard. From my understanding, your goal eventually is that the server does not download any certificate until a client connects. Only then it will start downloading certificates.

I can understand why this makes sense for a lot of use cases. But there should be a configuration option to decide which of these two different approaches the user would like to use.

Same goes with features such as "deduplication" btw. As a client that only needs the certificate for monitoring new domains, deduplication is definitely beneficial. For clients that want to monitor CA activity, there should not be any deduplication. Otherwise we're missing out on some activity.

Apart from all that, thank you for the PR. Looks good now with the changes applied :)