feat(auth): support custom scheme of http auth

[archerzz]

Clear and concise description of the problem

See cadl-ranch test case: https://github.com/Azure/cadl-ranch/blob/2fb006d5b4609647138382bf5d248a2edbde7742/packages/cadl-ranch-specs/http/authentication/http/custom/main.tsp#L10

scheme: "SharedAccessKey"

However, currently for auth of type: "http", we only explicitly support scheme of basic or bearer:

typespec/packages/http/src/types.ts

Lines 68 to 84 in 1f82d65

	export interface BasicAuth extends HttpAuthBase {
	type: "http";
	scheme: "basic";
	}
	/**
	* Bearer authentication (also called token authentication) is an HTTP authentication scheme that involves security tokens called bearer tokens.
	* The name “Bearer authentication” can be understood as “give access to the bearer of this token.” The bearer token is a cryptic string, usually generated by the server in response to a login request.
	* The client must send this token in the Authorization header when making requests to protected resources:
	* ```
	* Authorization: Bearer <token>
	* ```
	*/
	export interface BearerAuth extends HttpAuthBase {
	type: "http";
	scheme: "bearer";
	}

Downstream codes can actually support custom scheme, but the type is "never".
https://github.com/Azure/autorest.csharp/blob/0018fa9139bc21d79a258f67c1f981aaa3852b24/src/TypeSpec.Extension/Emitter.Csharp/src/lib/service-authentication.ts#L34-L49

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