What would you like to be added?

https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issue-search/#api-rest-api-3-search-post

Add the POST version of the issue search API.

Why is this needed?

The current j.client.Issue.SearchWithContext() function uses the GET query, which limits the JQL query to 2000 characters.

Anything else we need to know?

Not really, this would be <20 lines max depending on implementation. In the simplest case, we can check the query length and auto-switch to POST if it's >=2000, add a method param, or we can just copy the implementation of the current one and name the function j.client.Issue.SearchThroughPostWithContext() or something.