Use `Query` type instead of `str`

[ikhoon]

#14 introduced Query type. The existing APIs that use a string as a query expression need to be updated such as. https://github.com/line/centraldogma-python/blob/6098f1afe6b181388ab32bd7f91161dd7bd2a70d/centraldogma/content_service.py#L44 https://github.com/line/centraldogma-python/blob/6098f1afe6b181388ab32bd7f91161dd7bd2a70d/centraldogma/query.py#L30-L45