aconfig: add proto `bug` field

Add a `bug` field on the flag_declaration and parsed_flag proto
messages. This field is optional in the sense that it can occur zero or
more times, and aconfig will simply pass any value through.

Bug fields are included in the aconfig dump format, which can be
processed by other tools.

Also unify how protos.rs checks that fields marked 'optional' in the
proto file, but in practice are 'required', are actually set.

Test: atest aconfig.test aconfig.test.java
Bug: 288261336
Change-Id: I93de0005674822c6ff4d699bdc2c6509763a7f7f