Support per-connection OAuth credential overrides for multiple instances of the same source type

[Vai Ignatavicius (vai-airbyte)]

Problem / Current Behavior

When multiple connections exist for the same source type within a single workspace, OAuth credentials are scoped at the workspace level (workspace_id + source_type). This means configuring OAuth for a new connection
overrides the credentials for all existing connections of that source type, causing authentication failures (e.g., app_not_found errors) on previously working connections.

Requested Enhancement

OAuth credential overrides should be scoped per-connection (or at minimum per-source instance) rather than per-workspace. This would allow multiple independent OAuth credentials for the same source type to coexist within a
single workspace.

Use Case

Users who connect to multiple distinct instances of the same SaaS platform (e.g., multiple Salesforce orgs, multiple HubSpot portals) in a single workspace are currently blocked from doing so. The only workaround is to create
a separate workspace per connection, which is operationally burdensome and does not scale.

Additional Context

The workspace-level OAuth override API is documented here: https://reference.airbyte.com/reference/workspaceoauthcredentials