Create evlog Adapter

Add a new built-in adapter to evlog. Every adapter follows the same architecture and is built on the public toolkit primitives in evlog/toolkit, so a community adapter has the same shape as a built-in one.

PR Title

feat({name}): add the {Name} drain adapter

Recent examples: feat(loki): add the Grafana Loki drain adapter, feat(clickhouse): add the ClickHouse drain adapter. Use the adapter name as the conventional-commit scope, and register that scope (see touchpoint 11).

Scope timing caveat: the semantic PR check reads its scope list from the base branch, so a brand-new scope can't validate the very PR that introduces it. Either register the scope in a small preceding PR (the Loki/ClickHouse pattern), or use an unscoped title (feat: add the {Name} drain adapter) on the introducing PR.

Touchpoints Checklist