add microcosm

microcosm-rs consists of:

several services

• constellation: atproto interactions backlink index
• spacedust: global atproto interactions firehose
• slingshot: edge cache for atproto records and identities
• ufos: Timeseries stats and sample records for every collection ever seen in the atproto firehose

Libraries:

• links (wip): Rust library for parsing and extracting links (at-uris, DIDs, and URLs) from atproto records.

Microcosm is an important staple in the atproto dev community and used to power the underlying infra of many projects such as alternative bsky.app clients such as witchsky and red-dwarf.