feat: search directly from pgstac

[gadomski]

Description

Search directly into pgstac via a Rust function call or from the CLI. E.g. on an empty database:

$ docker compose up -d
$ cargo run -F pgstac search postgresql://username:password@localhost:5432/postgis
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.28s
     Running `target/debug/rustac search 'postgresql://username:password@localhost:5432/postgis'`
{
  "type": "FeatureCollection",
  "features": [],
  "links": [],
  "numberReturned": 0
}

Also fixes an issue where we weren't getting the next and prev tokens correctly in some cases.

Related issues

• Closes Search pgstac #515

Checklist

Delete any checklist items that do not apply (e.g. if your change is minor, it may not require documentation updates).

• Unit tests
• Documentation, including doctests
• Pull request title follows conventional commits
• Pre-commit hooks pass (prek run --all-files)