feat(storage): add named S3-compatible backends via s3+<name>:// URIs

[papaharry]

Summary

• Adds `storage.s3.named.` config map so a single Quickwit cluster can address multiple S3-compatible backends (separate creds, region, endpoint, checksum settings, flavor flags per entry).
• URIs of the form `s3+://bucket/path` route to the named backend; plain `s3://` continues to use the primary endpoint.
• Preserves the `s3+` qualifier through `Uri::parse_str` normalization so the storage resolver can recover the backend name on deserialization. Without this, named URIs were silently rewritten to plain `s3://` and resolved to the primary endpoint (silently wrong routing).

Why

The PoC needed A/B comparisons across Civo S3, SeaweedFS S3 (in-cluster), and OVH Object Storage on the same Quickwit cluster. Upstream Quickwit supports a single S3-compatible endpoint per cluster. This change makes multi-backend addressable per-index without forking the storage layer.

Changes

• `quickwit-config/src/storage_config.rs`: `NamedS3StorageConfig` struct + `storage.s3.named` BTreeMap, projection back to `S3StorageConfig` via `as_s3_config()`, redact + Debug + checksum_algorithm passthrough.
• `quickwit-common/src/uri.rs`: preserve `s3+` scheme through parse_str (so the named qualifier survives serde roundtrip via the metastore).
• `quickwit-storage/src/object_storage/s3_compatible_storage_resolver.rs`: `parse_named_key`, per-name cached S3Client, dispatch in `resolve()`.
• `docs/configuration/storage-config.md`: list `s3+://` in the URI protocols + Named S3 backends section with YAML example.

Tests

• `uri::tests::test_uri_s3_named_preserved` — parse + serde roundtrip preserves the qualifier
• `storage_config::tests::test_storage_s3_named_backends_serde` — YAML deser, `as_s3_config` projection
• `storage_config::tests::test_storage_s3_named_backends_redact`
• `s3_compatible_storage_resolver::tests::test_parse_named_key`

Test plan

• `cargo test -p quickwit-common --lib uri::`
• `cargo test -p quickwit-config --lib storage_config::`
• `cargo test -p quickwit-storage --lib s3_compatible_storage_resolver::`
• End-to-end smoke against a running cluster: created indexes with `s3+seaweedfs://`, `s3+ovh-morocco://`, `s3+civo-fra1://` URIs; verified ingest/search routes to the correct backend; verified URIs are stored intact in the metastore.

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