feat: add document loader (#9)

* add document loader

* add doc saver and tests

* update CUJ

* update test and CUJ

* update test

* update tests

* update black lint

* fix mypy

* address comments and update documentations

* update lint and tests

* update naming

* address comments

* update loader formatting

* update doc

* add user agent

* update user agent saver and primary key

* rebase

* lint

* Update integration.cloudbuild.yaml

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Co-authored-by: Averi Kitsch <[email protected]>

Author: Yuan325