Centralize the credential exportation in a single action on user menu

When users wants to export their credentials with many options and in different formats, the user needs to leave the panel the user is using to access the API ACCESS panel to then be able to export all his/her credentials.

The proposal is to create a pop-up modal that lists all the credentials the user can export and allow the user to select any one and download it. This modal could also allow options to copy the credentials to the clipboard instead of downloading it to a file.

The modal should be accessible via link in the User's Menu at the header bar in Horizon, so it will be accessible from any panel the user is, removing the necessity of changing contexts to get the credentials and improving the user's experience.

If the user is logged through Identity Federated (Using an Identity Provider), this popup should also allow the user to export the federated credentials file to log in Keystone via Federation, in this case, the credentials are going to have some placeholders to user set the Identity Provider credentials E.g. (username/password)