* auth/aws: Make identity alias configurable
This is inspired by #4178, though not quite exactly what is requested
there. Rather than just use RoleSessionName as the Identity alias, the
full ARN is uses as the Alias. This mitigates against concerns that an
AWS role with an insufficiently secured trust policy could allow an
attacker to generate arbitrary RoleSessionNames in AssumeRole calls to
impersonate anybody in the Identity store that had an alias set up.
By using the full ARN, the owner of the identity store has to explicitly
trust specific AWS roles in specific AWS accounts to generate an
appropriate RoleSessionName to map back to an identity.
Fixes#4178
* Respond to PR feedback
* Remove CreateOperation
Response to PR feedback
