Investigate benefits of switching to the new protobuf opaque API

[cmacknz]

See https://go.dev/blog/protobuf-opaque for details

[belimawr]

Reading the blog post, I believe we should migrate, accessing protobuf struct fields is very error prone (as shown in the blog post), the new opaque API prevents this while improving performance and paving the way for even more improvements in the future.

It's much easier and safer to have one consistent and safe API than expecting we won't write make mistakes while using proto structs or that they will be caught during reviews.

[swiatekm]

I'm not sure what the impact would be. Do we get proto related bugs often? Is decoding messages a significant part of agent's or fleet-server's resource footprint in managed mode?