proto: Add Pcrs.PrettyFormat
proto: Add Pcrs.PrettyFormat

Adds in a tpm2_readpcr style formatting for the PCR proto. Having this
be a method on the Pcrs type allows users to use this formatting outside
of the commandline tool.

Signed-off-by: Joe Richey <24d3dd8348b5cac19a417e52b4f8141ba91506f0@google.com>

google/go-tpm-tools,google/go-tpm-tools,google/go-tpm-tools