Add jitter correction for rate limit handling

[imjaroiswebdev]

Add jitter correction to ratelimit headers handling, additionally addresses case when no headers were present, but 429 error is received.

Introduced new test cases...

$ make test TESTARGS="-v -run TestHandleRatelimitError"
==> Testing go-pagerduty
=== RUN   TestHandleRatelimitErrorWithRatelimitHeaders
2023/12/11 21:33:16 ===== PagerDuty Cache Skipping Init =====
[INFO] Rate limit hit, throttling by 2.6 seconds until next retry to GET: http://127.0.0.1:51483/teams
--- PASS: TestHandleRatelimitErrorWithRatelimitHeaders (2.64s)
=== RUN   TestHandleRatelimitErrorNoRatelimitHeaders
===== PagerDuty Cache Skipping Init =====
[INFO] Rate limit hit, throttling by 6.4 seconds until next retry to GET: http://127.0.0.1:51485/teams
--- PASS: TestHandleRatelimitErrorNoRatelimitHeaders (6.38s)
PASS
ok      github.com/heimweh/go-pagerduty/pagerduty       9.262s
testing: warning: no tests to run
PASS
ok      github.com/heimweh/go-pagerduty/persistentconfig        0.319s [no tests to run]