move CredentialsCache get out from handle fetch result

[luoyuxia]

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Description

https://github.com/apache/fluss-rust/pull/143/files#diff-540a310c6f8139ea4592281917afecdd364792ffc3655a65221307465cabd09eR725

currently, when fetch response return to fetch from remote, it'll get credential from cache, as well as send credential get request. But we had better to move it out from the handle fetch result. We can follow java client did, which make the credential refresher as a background thread to refresh remote token.

Willingness to contribute

• I'm willing to submit a PR!