Add API for POST /user_directory/search
This takes a JSON body of the form:
```json
{
"term": "search term",
"limit": 42,
}
```
where "term" is the term to match against user IDs, display names and domains and "limit" is the maximum number of results to return.
The response body looks like
```json
{
"results ": [
{ "user_id": "@someid:mydomain.com", "display_name": "Some Name", "avatar_url": "mx://..." },
...
],
"limited": false
}
```
where "limited" indicates whether the "limit" was used to truncate the list.
