feat: blockbuilder component

[owen-d]

[WIP] Draft.

Introduces a "slimgester" component, which is essentially an ingester without write ahead logs, querying support (inc inverted indices or in-mem TSDB implementation), or limits application. It consumes a "job", aka(partition, offset_range) in kafka, writing our pre-existing format (chunks) to object storage. When finished, a TSDB is created from these chunks and written to storage.

This creates a way to idempotently consume a job specification and commit it in a more optimized form to a storage backend. We'll use this to iterate later on new storage formats, a scheduler+worker architecture for building, etc.

See plan.txt in this PR for more detail.