R1:fe01949a5c10

Add a Nuance GitHub repository source and basic polling

Summary: Ref T10537. Ref T10538. This calls GitHub, sorta?

Test Plan:
```
$ ./bin/nuance import --source poem
<cursor:events.repository> Polling GitHub Repository API endpoint "/repos/epriestley/poems/events".
<cursor:events.repository> This key has 4,988 remaining API request(s), limit resets in 1,871 second(s).
<cursor:events.repository> ETag for this request was ""4abdd3d66ad5ca38f5117b094e76f4ba"".
array(4) {
[0]=>
array(7) {
["id"]=>…