feat: add support for custom user agent

[jackwotherspoon]

Add support for custom user agent via user agent comments.

HTTP user agent's take the form of <product> / <product-version> <comment>

Previously, we did not allow support for the comment field, leaving no way
to customize a user agent:

cloud-sql-nodejs-connector/src/sqladmin-fetcher.ts

Lines 110 to 114 in 5bbd6be

	userAgentDirectives: [
	{
	product: 'cloud-sql-nodejs-connector',
	version: 'LIBRARY_SEMVER_VERSION',
	},

This PR changes that by adding userAgent as a supported ConnectorOptions
which will be passed and used to set the comment field, allowing customizations
to the user agent.

Closes: #401
Ref: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent