Seprate out login & registeration POST hooks

[yindia]

Is your feature request related to a problem? Please describe.
In current Implementation we are using common post hooks for login & registration. in #139 we face same issue where we have to implement POST hook for only login not for registration. Currently we use a blank ExecuteRegistrationPostHook but that is extra ideally we have to handle it separately

Describe the solution you'd like

each hook should be handled individually. registry_default_login.go
instead of using the postHook interface here, we should just iterate over []interface which has all of the hooks, and then do a type assertion

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.

[yindia]

one solution can be this

#registry_default_login.go
func (m *RegistryDefault) PostLoginHooks(credentialsType identity.CredentialsType) []login.PostHookExecutor {
	a := m.hooksPost(credentialsType, m.c.SelfServiceLoginAfterHooks(string(credentialsType)))
        a = append(a,hook.NewSessionDestroyer(m))
        b := make([]login.PostHookExecutor, len(a))
	for k, v := range a {
		b[k] = v
	}
	return b
}
# registry_default_registration.go
func (m *RegistryDefault) PostRegistrationHooks(credentialsType identity.CredentialsType) []registration.PostHookExecutor {
	a := m.hooksPost(credentialsType, m.c.SelfServiceRegistrationAfterHooks(string(credentialsType)))
        // add hooks if any in case of registration
	b := make([]registration.PostHookExecutor, len(a))
	for k, v := range a {
		b[k] = v
	}
	return b
}

Default registry hook only have common hooks like redirect and sessionIssuer

[nmlc]

@evalsocket can I grab this one? Seems like small enough issue to start with :)

[yindia]

@nmlc discuss it with @aeneasr before starting.