Add --group-id flag to assume a group during OAuth login

[grundprinzip]

Why

Wire the SDK's u2m.WithAssumeGroup option into databricks auth login so users can assume a Databricks group during the U2M OAuth flow. When set, the numeric group ID is sent as the assume_group query parameter on the authorize request and the minted token is scoped to that group.

Changes

• Add a --group-id flag to auth login. The value follows the same precedence as --scopes: an explicit flag wins, otherwise re-login preserves the group ID from the existing profile.
• Wire WithAssumeGroup into both the standard and discovery login flows and persist the group ID to the profile as group_id.
• Add AssumeGroupID to the profile struct and its file parsing so re-login can read the previously configured group ID back.
• Skip group_id in the env loader's always-skip list so it comes from the selected profile only, consistent with other auth-steering fields.

Co-authored-by: Isaac

[github-actions]

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Inputs:

• PR number: 5822
• Commit SHA: ed308ca833590190f8c9f5d80294fab8c052f50f

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[github-actions]

Approval status: pending

/cmd/auth/ - needs approval

Files: cmd/auth/login.go, cmd/auth/login_test.go
Suggested: @simonfaltum
Also eligible: @tanmay-db, @renaudhartert-db, @mihaimitrea-db, @tejaskochar-db, @Divyansh-db, @hectorcast-db, @parthban-db, @chrisst, @rauchy

/libs/databrickscfg/ - needs approval

Files: libs/databrickscfg/loader.go, libs/databrickscfg/profile/file.go, libs/databrickscfg/profile/profile.go
Suggested: @simonfaltum
Also eligible: @tanmay-db, @renaudhartert-db, @mihaimitrea-db, @tejaskochar-db, @Divyansh-db, @hectorcast-db, @parthban-db, @chrisst, @rauchy

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose) can approve all areas.
See OWNERS for ownership rules.}