net/rubygem-beefcake: new port - Protocol Buffers library in pure Ruby

Straightfoward pure-ruby first-class Protocol Buffers library. Supports
optional and required fields, repeated and packed repeated fields,
varints, 32 and 64 bit fields, length-delimited fields, embedded
messages and enums, and defaults. Per spec, unknown fields are ignored.

WWW: https://github.com/protobuf-ruby/beefcake

Reviewed by: romain
Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D17710