Improve executors and authentication services

[ThaminduDilshan]

Problem

Currently all executors reside in respective sub packages in the internal/executor package. Some of these executors need to utilize internal services. Currently auth executors invoke individual auth services in internal/authn/ packages.
These individual services need to be injected to the executors with future refactoring. Ideally these auth executors should belong to the respective auth packages.

Suggested Improvement

Rather than creating all authn services in the servicemanager level, we can follow the below approach.

In the servicemanager.go, we will Initialize the authentication service with all the needed services

# servicemanager.go
authnService, executorProvider = authn.Initialize(
		mux,, userService
		idpService, jwtService
	)

....

_ := flowExec.Initialize(mux, executorProvider)

And we will have a authn/init.go, we will Initialize each of these authenticaiton package which will return authenticaiton service and corresponding executor. Using that we can then create authnService and executorProvider(I am not sure whether this name) which will keep the map of executors which will be used for the flowexec

# authn/init.go
   ....
	oauthAuthnSvc, oAuthExectuor := authnoauth.Initialize(idpService, userService)
	oidcAuthnSvc, oidcExectuor := authnoidc.Initialize(idpService, userService, jwtService)
   ......
    authnService := newAuthenticationService(
		...
		oauthAuthnSvc,
		oidcAuthnSvc,
		....
	)
   ...
   
   executorProvider := ...

  return authnService, executorProvider

In the authn/oauth/init.go, we can Initialize each of authenticaiton service and corresponding executor as well and return both.

# authn/oauth/init.go
 oauthAuthnSvc := newOAuthAuthnService(...)
 oauthAuthExceutor := newOAuthAuthExceutor(oauthAuthnSvc)
return oauthAuthnSvc, oauthAuthExceutor

So with this change there won't be need for the executor package.

I like this approach because,

1. It keeps the main servicemanager clean, it doesn't need to know about individual authentication service
2. It's better if we can keep the authentication service and corresponding executor under the same package
3. This will give us add recoveryservice and it's executors in another package and feed into the flowExce.

---

The directory structure will be like this:

authn/
    oauth2/ 
        service.go
        oauthexecutor.go
        init.go
    github/ 
        service.go
        githubexecutor.go
        init.go
   service.go
   handler.go
   init.go

Originally suggested by @senthalan in #673 (comment)