This change allows the admin container to be platform agnostic as it
retrieves its .ssh/authorized_keys from a base64-encoded JSON block in
user-data rather than relying on the AWS instance metadata service
(IMDS) directly.

Example JSON: {"ssh":{"authorized_keys":[]}}

This allows users who want to set TrustedUserCAKeys to do so by adding
them as a base64-encoded JSON block in user-data.

Example JSON: {"ssh":{"trusted_user_ca_keys":[]}}

This commit bumps the version from v0.5.2 to v0.6.0 and adds a changelog
for the admin container.