[CDAP-19183] Switch Placeholder Credential to External Credential Type

[dli357]

This is necessary because if internal auth is enabled and credential propagation is disabled, a default placeholder credential is used for propagation. This causes internal auth to break because it assumes that the credentials passed to it are proper base64-encoded access tokens.

Update: instead of always enabling credential propagation (which is brittle and would not work for non-credential-propagating architectures), we go ahead and assign an external placeholder credential instead of an internal placeholder credential so that the access enforcer extension can still perform enforcement based on user ID.

[gitpod-io[bot]]

[dli357]

Update: instead of always enabling credential propagation (which is brittle and would not work for non-credential-propagating architectures), we go ahead and assign an external placeholder credential instead of an internal placeholder credential so that the access enforcer extension can still perform enforcement based on user ID.