fix(API): Support for auth api oidc provider

[wooj2]

This PR largely mirrors the following PR despite a couple naming changes:
#840

This PR allows customers to inject their own OIDC Auth Provider for use with the API category.

Customer facing call site would look like (owned by customer):

            let apiAuthProviderFactory = MyAPIAuthProviderFactory()
            let dataStorePlugin = AWSDataStorePlugin(modelRegistration: AmplifyModels())
            try Amplify.add(plugin: dataStorePlugin)
            try Amplify.add(plugin: AWSAPIPlugin(apiAuthProviderFactory: apiAuthProviderFactory))
            try Amplify.configure()

Impl ofMyAPIAuthProviderFactory (owned by customer)

class MyAPIAuthProviderFactory: APIAuthProviderFactory {
    let myAuthZeroAuthProvider = MyAuthZeroOIDCAuthProvider()
    
    override func oidcAuthProvider() -> AmplifyOIDCAuthProvider? {
        return myAuthZeroAuthProvider
    }
}

Impl of APIAuthProviderFactory (owned by AWS)

import Foundation

open class APIAuthProviderFactory {
    public init() {
    }
    open func oidcAuthProvider() -> AmplifyOIDCAuthProvider? {
        return nil
    }
}

public protocol AmplifyOIDCAuthProvider {
    func getLatestAuthToken() -> Result<String, Error>
}

In the event that we add a new authProvider type:

• No change to customer call site
• No change to customer owned subclass
• Required change to Amplify owned APIAuthProviderFactory

import Foundation

open class APIAuthProviderFactory {
    public init() {
    }
    open func oidcAuthProvider() -> AmplifyOIDCAuthProvider? {
        return nil
    }
+    open func apiKeyAuthProvider() -> AmplifyAPIAuthProvider? {
+        return nil
+    }
}

public protocol AmplifyOIDCAuthProvider {
    func getLatestAuthToken() -> Result<String, Error>
}

+public protocol AmplifyAPIAuthProvider {
+    func getAPIKey() -> Result<String, Error>
+}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.