[Cadl RLC] Fix the extensible enum issue and support apiVersion default value suggested from cadl-dpg

[MaryGao]

• Fix the extensible enum issue
• Support apiVersion default value suggested from cadl-dpg

In this pr we handle the cadl extensible enum as string literal, I think the more consistent way is handled as string. See our current type map test cases here.

previous implementation:

autorest.typescript/packages/cadl-rlc-test/test/anomalyDetector/cadl-output/src/anomalyDetector.ts

Lines 11 to 16 in 3548055

	export default function createClient(
	Endpoint: string,
	ApiVersion: "v1.1",
	credentials: KeyCredential,
	options: ClientOptions = {}
	): AnomalyDetectorClient {

Current pr:

autorest.typescript/packages/cadl-typescript/test/unit/generate/clientFactory.spec.ts

Lines 188 to 197 in 93b44b6

	/**
	* Initialize a new instance of the class testClient class.
	* @param Endpoint type: string The endpoint to use.
	* @param Version type: string The version to use. Possible values: v1.1
	*/
	export default function createClient(
	Endpoint: string,
	Version: string,
	options: ClientOptions = {}
	): testClient {

draft pr is here: #1664