[IMP] hr_appraisal_oca: add configurable email notifications

[ebauza]

Description

This PR adds a configurable option to disable email notifications
in the appraisal workflow.

A new boolean field "Send email notifications" allows managers
to control whether automatic emails are sent during appraisal
confirmation and completion.

Functional behavior

• By default, email notifications are enabled.
• When disabled, no emails are sent during confirmation or completion.
• All other workflow behavior remains unchanged (state transitions,
  activities, etc.).
• This configuration is only visible to managers.

Technical details

• Added field send_email_notifications on hr.appraisal
• Introduced helper method _can_send_appraisal_email
• Updated _send_email to respect the configuration and context override

Testing

• Added tests to validate:
  • notifications enabled → emails are sent
  • notifications disabled → no emails are sent
  • context override behavior
• Existing tests remain unchanged and passing

Rationale

This improvement provides flexibility to avoid sending automatic
emails in specific appraisal cases without altering the standard workflow.

[OCA-git-bot]

Hi @pedrobaeza,
some modules you are maintaining are being modified, check this out!