Added builder and moved config class as an inner class of GemFireVectorStore to match other vector store projects

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Comment From: sobychacko

@jhuynh1 As a general comment, please add your name as an author to all the classes you modified.

Comment From: sobychacko

@jhuynh1 Due to the latest changes in main, this PR branch has diverged from the main, and I had merge conflicts on rebase. Started from scratch based on your changes and addressed the review comments. The changes are here. Closing the PR.