Export Client interface for extensibility

[FumingPower3925]

Problem

The internal rawClient interface is unexported:

type rawClient interface {
    doRequest(ctx context.Context, workerID int) (int, error)
    close()
}

Users cannot implement custom protocol handlers (gRPC, QUIC, WebSocket) to use with the benchmarker.

Solution

Export the interface as Client:

// Client is the interface for protocol-specific benchmark clients.
// Implement this interface to add support for custom protocols.
type Client interface {
    // DoRequest sends a single request and returns the response status code.
    DoRequest(ctx context.Context, workerID int) (int, error)
    // Close releases all resources held by the client.
    Close()
}

• Rename internal usages from rawClient to Client
• Add Config.Client Client field for user-provided implementations
• When Config.Client is set, skip built-in H1/H2 client creation

Acceptance Criteria

• Client interface is exported
• Config.Client field allows custom implementations
• Built-in H1/H2 clients implement Client
• Example in godoc showing custom client usage

Breaking Change

Renames internal interface. No external breakage since interface was unexported, but the new Config.Client field is additive.