feat: port GDCH credentials support to Node.js Auth SDK

[macastelaz]

Add support for GDCH Credentials to the Node.js Auth SDK

Fixes #8289

[gemini-code-assist]

Code Review

This pull request introduces the GdchClient to support Google Distributed Cloud Hosted (GDCH) credentials, including token exchange logic using JWT assertions and integration into the GoogleAuth class. Feedback focuses on optimizing performance by using asynchronous file operations for CA certificates and replacing new Date().getTime() with Date.now() for consistency.

[gemini-code-assist]

Using fs.readFileSync in an async method blocks the event loop. It is recommended to use the asynchronous fs.promises.readFile instead. Additionally, consider caching the https.Agent or the CA certificate buffer to avoid re-reading the file and re-creating the agent on every token refresh.

Suggested change

	const ca = fs.readFileSync(this.caCertPath);
	requestOpts.agent = new https.Agent({ ca });
	const ca = await fs.promises.readFile(this.caCertPath);
	requestOpts.agent = new https.Agent({ ca });

[gemini-code-assist]

Use Date.now() instead of new Date().getTime() for better performance and consistency with other parts of the file (e.g., line 223).

Suggested change

	tokens.expiry_date = new Date().getTime() + tokenResponse.expires_in * 1000;
	tokens.expiry_date = Date.now() + tokenResponse.expires_in * 1000;