return partnerId in invitations post response

[chriamue]

Signed-off-by: Christian M chriamue@gmail.com

[chriamue]

This merge requests adds the partnerId to the response while creating a new invitation, which allows later to find the partner based on the created invitation.
eg:

# POST /api/invitations
# response body
{
    "invitationUrl": "http://bpa-agent1:8030?oob=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",
    "invitationId": "258e9b21-0dad-4cd2-9aac-22450b98202b",
    "partnerId": "b9b08bab-3726-4554-8d8c-2bb456bf1642"
}