Reduce number of log messages produced by default Client

[chrisnovakovic]

retryablehttp.Client logs excessively (one Printf to stdout per HTTP request) unless a retryablehttp.LeveledLogger is configured as its logger. Add a logger that implements retryablehttp.LeveledLogger, whose verbosity defaults to INFO and can be controlled via the VERBOSITY environment variable, and use it as the logger for the default Client.

The logger increases the level of some messages produced by retryablehttp.Client, in particular those that report when requests are about to be retried, for the sake of visibility.

[chrisnovakovic]

The test suite provides the best example of the reduction in log spam that this PR achieves.

Before:

=== RUN   TestIssueService_GetWatchers
2024/03/12 17:21:22 [DEBUG] GET http://127.0.0.1:45057/rest/api/2/issue/10002/watchers
2024/03/12 17:21:22 [DEBUG] GET http://127.0.0.1:45057/rest/api/2/user?accountId=000000000000000000000000
--- PASS: TestIssueService_GetWatchers (0.00s)

=== RUN   TestClient_Do_TooManyRequests
2024/03/12 17:21:22 [DEBUG] GET http://127.0.0.1:45433/
2024/03/12 17:21:22 [DEBUG] GET http://127.0.0.1:45433/ (status: 429): retrying in 3s (4 left)
--- PASS: TestClient_Do_TooManyRequests (3.01s)

After:

=== RUN   TestIssueService_GetWatchers
--- PASS: TestIssueService_GetWatchers (0.00s)

=== RUN   TestClient_Do_TooManyRequests
time=2024-03-12T17:03:12.482Z level=WARN msg="retrying request" request="GET http://127.0.0.1:36983/ (status: 429)" timeout=3s remaining=4
--- PASS: TestClient_Do_TooManyRequests (3.00s)