feat: implement trust boundaries

[bshaffer]

b/480120031

Implementation

• Impersonated Service Account Credentials
• Service Account Credentials and Service Account JWT Credentials
• Metadata Credentials
• External Account Authorized User Credentials (??)

Testing

• Impersonated Service Account Credentials (Staging)
• Service Account Credentials and Service Account JWT Credentials (Staging)
• Metadata Credentials (Blocked)
• External Account Authorized User Credentials (??)