Export getGraphqlServer and accept initialized backend

[robert-cronin]

Description of the PR

Fixes #2240

Some notes:

• I intended to restrict this PR to refactoring only instead of altering functionality, so if there is any change to functionality it was not intentional, and we should probably flag and revert.
• The main change here is: a refactor of getGraphqlServer to allow it to be used programmatically by out-of-tree backends, allows the backend to be passed in as-is without relying on parsing of the flags for config.

cc @ridhoq

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If GraphQL schema is changed, GraphQL client updates/additions have been made
• If OpenAPI spec is changed, make generate has been run
• If ent schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[lumjjb]

LGTM, but request a slight refactor to expose this through a package instead of from the cmd package