add_watcher should accept a user ID, as per Jira API

[supermitch]

Problem trying to solve

I noticed that in version 3.5.x, the new Jira cloud (and this package) required a user ID to add watchers.
See https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issue-watchers/#api-rest-api-3-issue-issueidorkey-watchers-post

However, in #1682, someone added the (very useful) ability to once again add watchers by email addresses. However, they:

1. Added an extra API call
2. Broke consistency with the API docs

Possible solution(s)

Please update the method and documentation so we can pass either an email address or an accountId, if we already have it.

Alternatives

• Alternatively, I would revert the functionality to be more in-line with the REST API.
• Alternatively, update the documentation to be more explicit, stating that a user search is being conducted internally.

Additional Context

Source: https://jira.readthedocs.io/_modules/jira/client.html#JIRA.add_watcher

[dimitarOnGithub]

I agree with the sentiment here, and also described that the internal user lookup might be resulting in unexpected behaviour in this comment on #1301.

@adehad I'll be happy to look into this since I believe it'd be a good way to solve both this issue, as well as #1284.

[adehad]

@dimitarOnGithub happy for you to look further into a solution, thanks for this