Switch content/attachment id field to string format

Issue Summary

See https://jira.atlassian.com/browse/CONFCLOUD-73441 for the background information.

This ticket covers the switching of id field from number to string.

Steps to Reproduce

(high level)

1. Have an content with id > 2^53.
2. Have an app that listens for the webhooks.
3. Perform an operation with this content.
4. Receive a webhook in an app.

Expected Results

Content id is returned as a string

Actual Results

Content id is returned as a number

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available