refactor(backend): Relocate User Login info from User table to AuthProvider table

[Neilk1021]

What changes were proposed in this PR?

As discussed in #6716 we'd like to split authentication handling into its own table auth_providers. For future support of multiple sources of authentication (e.g. github, IEEE accounts, etc.). This PR migrate the Schema from the image on the left to the one on the right.

This PR:

1. Creates a new auth_providers table in texera_ddl.
2. Providers a migration script 29.sql to migrate old account data to the new schema.
3. Refactors backend to match new schema.
4. Refactors portions of GoogleAuthService to a generic ExternalAuthProvisioner to be used in future PRs adding new external logins.

Any related issues, documentation, discussions?

Closes #7048

How was this PR tested?

Compiled and ran all tests as well as deployed locally to verify functionality.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.8

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• Contributors with relevant context: @Yicong-Huang, @Ma77Ball, @mengw15
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