[ADD]pms: change related by compute in checkin name

[DarioLodeiros]

This pull request refactors how the name field is managed for the PmsCheckinPartner model. Instead of being a related field, name is now a computed and stored field, allowing for more flexible and accurate name handling, especially when partner_id is not set. The computation logic has also been expanded to derive the name from first and last name fields when necessary.

Field definition and computation improvements:

• Changed the name field in PmsCheckinPartner from a related field to a computed and stored field, using the _compute_name method for its value.
• Updated the _compute_name method to depend on partner_id, firstname, lastname, and lastname2, and to compute the name from these fields if partner_id is not set.

ATENTION: Need review pms_l10n_es & pms_fastapi_contact_lastname2 to take account lastname2 in the compute