fix: change the type of httpCli to HTTPClient

[holyachon]

PR about #11

I think type of httpClient in StoreClient is *http.Client, so user can't use those functions like SetResponseErrorHandler, SetRetry out of library code to wrap client.

I expect that after this change, user can write below code.

        httpClient := &http.Client{}
	client := SetRetry(httpClient, &JitterBackoff{}, ShouldRetryDefault)
	storeClient := NewStoreClientWithHTTPClient(&StoreConfig{...}, client)

Feel free to give your opinion that improves this PR or change the way of implementation.
Thank you.

[richzw]

Got it. Thank you very much for your detailed response and your PR.

LGTM.