feat: add X-Glean headers for experimental features and deprecation testing

[travis-hoover-glean]

Summary

• Add XGlean beforeRequest hook that sets X-Glean-Exclude-Deprecated-After and X-Glean-Experimental headers
• Introduce new SDK options WithExcludeDeprecatedAfter and WithIncludeExperimental with corresponding environment variable support
• Add documentation for testing experimental features and deprecation testing

This is the Go equivalent of gleanwork/api-client-typescript#95

Test plan

• Verify headers are set correctly when using SDK constructor options
• Verify headers are set correctly when using environment variables
• Confirm environment variables take precedence over SDK options
• Ensure no headers are set when neither option is configured
• Verify multiple SDK instances maintain separate configurations

Usage Example

import (
	"os"

	apiclientgo "github.com/gleanwork/api-client-go"
)

// Using SDK options
s := apiclientgo.New(
	apiclientgo.WithSecurity(os.Getenv("GLEAN_API_TOKEN")),
	apiclientgo.WithInstance(os.Getenv("GLEAN_INSTANCE")),
	apiclientgo.WithExcludeDeprecatedAfter("2026-10-15"),
	apiclientgo.WithIncludeExperimental(true),
)

// Or using environment variables
// export X_GLEAN_EXCLUDE_DEPRECATED_AFTER="2026-10-15"
// export X_GLEAN_INCLUDE_EXPERIMENTAL="true"