fix: move X-Glean header config to preserved hooks package

[chris-freeman-glean]

Summary

Fixes the daily CI Generate workflow failures that started after #66 was merged.

Problem

PR #66 added custom fields (excludeDeprecatedAfter, includeExperimental) to SDKConfiguration.java and Glean.java. However, these files are auto-generated by Speakeasy and marked "DO NOT EDIT". When the SDK regenerates, these custom fields are removed, but XGleanHeadersHook.java (in the preserved hooks/ package) still references them:

error: cannot find symbol - config.excludeDeprecatedAfter()
error: cannot find symbol - config.includeExperimental()

Solution

Move the custom configuration to a dedicated class in the preserved hooks/ package:

• GleanCustomConfig.java - Thread-safe singleton storing custom config values
• GleanBuilder.java - Builder wrapper that delegates to Glean.Builder but adds excludeDeprecatedAfter() and includeExperimental() methods that write to the singleton
• XGleanHeadersHook.java - Updated to read from GleanCustomConfig instead of SDKConfiguration

Users now use GleanBuilder.create() instead of Glean.builder() when they need the custom options:

import com.glean.api_client.glean_api_client.hooks.GleanBuilder;

Glean glean = GleanBuilder.create()
        .apiToken("token")
        .instance("instance-name")
        .excludeDeprecatedAfter("2026-10-15")
        .includeExperimental(true)
        .build();

Environment variables continue to work unchanged.

Test plan

• ./gradlew build passes
• ./gradlew test --tests "XGleanHeadersHookTest" passes
• Verified fix survives speakeasy run regeneration