feat: add custom headers support via .header() on client builder

[generall]

Adds the ability to set custom headers on the Qdrant client so they are sent with every request (gRPC and REST).

Changes

• QdrantConfig (config.rs): New custom_headers: Vec<(String, String)> field and .header(key, value) builder method. Can be chained to add multiple headers.
• TokenInterceptor (auth.rs): Injects custom headers into gRPC request metadata (after api-key).
• download_snapshot (snapshot.rs, download_snapshots feature): Builds the REST request with a reqwest::Client and adds api_key + custom headers instead of plain reqwest::get().

Usage

use qdrant_client::Qdrant;

let client = Qdrant::from_url("http://localhost:6334")
    .header("x-custom-id", "my-client")
    .header("x-request-source", "batch-job")
    .api_key(std::env::var("QDRANT_API_KEY"))
    .build()?;

Custom headers are sent on all gRPC calls and on snapshot download (REST) when using the download_snapshots feature.

Made with Cursor