[CSA Importer] Read CNECs borders

[bqth29]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What is the current behavior?
CNECs were always created with a null border.

What is the new behavior (if this is a feature change)?
According to what was decided for the CSA SWE process, the border to which an AssessedElement is linked can be specified though the field AssessedElement.OverlappingZone.

Only REE needs to specify this information since REN and RTE both only have one border so the AssessedElement's operator is enough to know said border.

A new CSA CRAC creation parameters field called "borders" was added to indicate how to map an AssessedElement to a specific border.

"borders": [
  {
    "name": "ES-FR",
    "eic": "10YDOM--ES-FR--D",
    "default-for-tso": "RTE"
  },
  {
    "name": "ES-PT",
    "eic": "10YDOM--ES-PT--T",
    "default-for-tso": "REN"
  }
]

• name is the name of the border that will appear in the border field of the CNECs in the CRAC
• eic is the EI Code of the border
• default-for-tso indicates for which operator (TSO) this border is the default border (used for TSOs with only one border)

When an AssessedElement is being parsed by the importer the OverlappingZone field is read:

• if it is not null then its border is the border with the same EI Code (or null if no such border were defined in the CRAC creation parameters)
• if it is null, then its border is the default border for the AssessedElement's operator (or null is there is no default border for this operator)

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No