Add shell command to dump certificate chains

As an aid to debugging and development, introduce a shell command to
print a certificate chain that has been provisioned to a given component
on the device.

The certificate chain is printed as a series of PEM-encoded X.509
certificates from leaf to root.

Test: adb shell cmd remote_provisioning certify default
Test: atest RemoteProvisioningShellCommandTest
Change-Id: I32840aa0d30157f0668a1a37abab24a01f83a750