Support consumer-supplied HttpClient instance

[rammrain]

Summary

Allow SDK consumers to supply their own java.net.http.HttpClient instance to MontonioHttpClient, enabling shared connection pools, custom proxy/TLS configuration, and other HttpClient customizations.

Background

During the design of #11, consumer-supplied HttpClient was explicitly deferred (design decision #1) to keep the initial implementation simple. The package-private constructor already accepts an HttpClient parameter for testability — this issue is about making that capability part of the public API.

Requirements

• Make the MontonioHttpClient(MontonioSdkConfiguration, HttpClient) constructor public
• When a consumer supplies their own HttpClient, connectTimeout from configuration is not applied (the consumer's HttpClient settings take precedence)
• The single-arg constructor remains the default path, creating an internal HttpClient with configuration-based timeouts
• Document the two construction modes

Refs

• docs/plans/2026-04-10-http-client-design.md — design decision Initial setup #1
• initial-docs/project2.md — "The SDK must allow the consumer to supply their own HTTP client or provide a default"

Testing

• Verify consumer-supplied HttpClient is used for requests
• Verify default constructor still creates its own HttpClient with connectTimeout