feat: option to log time spent in S3Storage calls

[jennifer-richards]

Adds an option, ietf_log_blob_timing, to the CustomS3Storage class. When True, emits logs like:

INFO: datatracker:48: S3Storage timing: _open('draft-richards-fake-draft-01.txt', ...) for staging took 0.044395
INFO: datatracker:48: S3Storage timing: delete('draft-richards-fake-draft-01.txt') for staging took 0.002906
INFO: datatracker:48: S3Storage timing: _open('draft-richards-fake-draft-01.html', ...) for staging took 0.00201
INFO: datatracker:48: S3Storage timing: delete('draft-richards-fake-draft-01.html') for staging took 0.001693
INFO: datatracker:48: saved file /assets/www6s/staging/draft-richards-fake-draft-01.xml
INFO: datatracker:48: S3Storage timing: _save('draft-richards-fake-draft-01.xml', ...) for staging took 0.007767