Add operation level servers support for java okhttp-gson client

Created by: ajrice6713

Add operation level server support for Java okhttp-gson http client

Closes #10847 (closed)

At the API level - user can specify a custom base URL. If no custom base url is specified, the API selects the 1st path level server (if any), unless a different host index is specified. If no operation level servers are defined, the http request is built using the spec default server url.

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