feat : Added get all Invitation endpoint

[marveldo]

Added an endpoint where a superadmin can get all the invitations to an organisation and wrote a test scripts to test my new changes

Description

This new addition adds a new endpoint to get all invitations in an organisation it is a get request and it returns a status code of 200 with a list of all organisation invites

​## Related Issue (Link to issue ticket)
Related Issue

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Motivation and Context

Motivation was to build an api endpoints in which all organisation invites can be retrieved

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How Has This Been Tested?

This changes was tested with the use of pytest and Post man

Screenshots (if appropriate - Postman, etc):

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