feat: add setLogger for custom logger support

[mlingner]

Summary

Attempts to address #84

This PR adds a setLogger() function that allows users to provide a custom logger implementation. This is a simpler implementation but follows the general pattern used by OpenTelemetry and other JS libraries.

This enables:

• Disabling ANSI color codes for cloud logging (GCP, etc.)
• Using custom logging libraries (Winston, Pino)
• Completely disabling logging

Changes

• Added setLogger(customLogger: Logger | null) to set a custom logger or disable logging
• Added getLogger() to retrieve the current logger instance
• Added NoOpLogger class for when logging is disabled
• Maintained backward compatibility - existing logger import still works
• Added tests

Usage Examples

Disable logging entirely:

import {setLogger} from '@google/adk';
setLogger(null);

Plain logger without ANSI colors (for GCP/cloud):

import {setLogger, Logger, LogLevel} from '@google/adk';

const plainLogger: Logger = {
  log: (level, ...args) => console.log(`[ADK ${LogLevel[level]}]:`, ...args),
  debug: (...args) => console.debug('[ADK DEBUG]:', ...args),
  info: (...args) => console.info('[ADK INFO]:', ...args),
  warn: (...args) => console.warn('[ADK WARN]:', ...args),
  error: (...args) => console.error('[ADK ERROR]:', ...args),
};
setLogger(plainLogger);

Use Winston for structured JSON logging:

import {setLogger, Logger} from '@google/adk';
import winston from 'winston';

const winstonInstance = winston.createLogger({
  format: winston.format.json(),
  transports: [new winston.transports.Console()],
});

const winstonLogger: Logger = {
  log: (level, ...args) => winstonInstance.log(['debug','info','warn','error'][level], args.join(' ')),
  debug: (...args) => winstonInstance.debug(args.join(' ')),
  info: (...args) => winstonInstance.info(args.join(' ')),
  warn: (...args) => winstonInstance.warn(args.join(' ')),
  error: (...args) => winstonInstance.error(args.join(' ')),
};
setLogger(winstonLogger);

Test Plan

• Run unit tests: npx vitest run core/test/utils/logger_test.ts
• Tested locally with all three scenarios above

[kalenkevich]

Thanks!

[mlingner]

Addressed merge conflicts.