[CHA-1558] support ext custom header

[rafaelmf3]

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• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

Summary

Adds support for configuring the x-stream-ext header in the Stream Chat Java SDK. This header allows clients to pass additional metadata (such as device information, app version, etc.) that will be included in the request_info.ext field for webhooks and other backend processing.

What Changed

• Added X_STREAM_EXT_PROP_NAME constant for the configuration property
• Updated extendProperties() method to read x-stream-ext value from environment variables or system properties
• Added getXStreamExt() helper method to retrieve the configured value
• Updated the HTTP interceptor in buildOkHttpClient() to include the X-Stream-Ext header in all requests when configured

Why

This feature aligns the Java SDK with other Stream Chat SDKs (JavaScript, Flutter, Python) that already support the x-stream-ext header. The backend expects this header and includes it in the request_info.ext field, which is useful for:

• Passing device information to webhooks
• Including app version metadata
• Custom client-side information for moderation/fraud detection
• Any other custom metadata needed by the backend

Usage

The x-stream-ext header can be configured in three ways:

**1. Via Properties object:**va
var properties = new Properties();
properties.put(DefaultClient.API_KEY_PROP_NAME, "");
properties.put(DefaultClient.API_SECRET_PROP_NAME, "");
properties.put(DefaultClient.X_STREAM_EXT_PROP_NAME, "device-id=123");
var client = new DefaultClient(properties);
DefaultClient.setInstance(client);