Dashboard Param Values Common Impl for Embedding/Preview Embedding (#42052)

* Dashboard Param Values Common Impl for Embedding/Preview Embedding

Refactor the embedding api namespace to allow a bit of reuse between the embedding api and the embedding preview
api. In some cases, the implementations are not identical (in terms of the shape of data expected/endpoints
available), so this is a first step to making them the same.

Related:

https://www.notion.so/metabase/Make-embedding-preview-behave-consistently-with-actual-static-embeds-855353d8e5e8411d8164c7ac563c7d2f



* Add a test to verify that preview-embed param-values endpoint works

* Use preview embed endpoint and fix relevant tests

* Add tests

* Change name to reflect what actually happens in the function

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Co-authored-by: Mahatthana Nomsawadi <mahatthana.n@gmail.com>