feat: Prevent duplicated subscribers creation

[patriciareinoso]

Description

We now validate whether a subscriber with the same name already exists before creating it. If it already exists, an error is displayed to the user in the creation modal.

There is an issue with the CURL GET of subscribers (omec-project/webconsole#109) in which an "empty" subscriber object is returned if the subscriber does not exists. So, as a workaround I added a check over one of the fields to be able to differentiate between a subscriber that exists and one that does not.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.