[Feature] proxy: implement token bucket load balancing for upstreams

[Feature] proxy: implement token bucket load balancing for upstreams

Add weighted load balancing algorithm that considers message size and
current backend load when selecting upstreams. Each upstream has a token
pool that gets depleted proportionally to message size and replenished
when requests complete successfully.

- Add RSPAMD_UPSTREAM_TOKEN_BUCKET rotation type
- Implement min-heap based selection for O(log n) upstream selection
- Reserve tokens proportional to message size (base_cost + size/scale)
- Return tokens on success (restores available) or failure (lost)
- Fall back to least-loaded upstream when all are token-exhausted
- Add UCL configuration: token_bucket { max_tokens, scale, min_tokens, base_cost }