Update sasl options to support AWS specific fields

[r115]

Adds optional fields to the SASLOption interface. #935

[Nevon]

I think we can do better than just adding optional fields, since these fields are only relevant for certain authentication mechanisms. Using conditional types and discriminating unions we can make the fields dependent on the mechanism:

type SASLMechanismOptionsMap = {
  'plain': { username: string, password: string },
  'scram-sha-256': { username: string, password: string },
  'scram-sha-512': { username: string, password: string },
  'aws': { authorizationIdentity: string, accessKeyId: string, secretAccessKey: string, sessionToken?: string },
  'oauthbearer': { oauthBearerProvider: () => Promise<OauthbearerProviderResponse> }
}

export type SASLMechanism = keyof SASLMechanismOptionsMap
type SASLMechanismOptions<T> = T extends SASLMechanism ? { mechanism: T } & SASLMechanismOptionsMap[T] : never
export type SASLOptions = SASLMechanismOptions<SASLMechanism>

It's a little verbose, but basically we map the mechanism to the desired options (SASLMechanismOptionsMap), then use that mapping to create a union type (SASLOptions) with all the possible keys of the map (SASLMechanismOptions<T>).

The result is that the fields are checked depending on the value of the mechanism, so you couldn't, for example, specify authorizationIdentity unless you are using the aws mechanism.

[r115]

@Nevon gotcha. Let me refactor.