feat: add proto3_optional to proto_custom_library

[noahdietz]

This adds the proto3_optional enablement flag to proto_custom_library
which in turn enables it in proto_library_with_info and micro-Bazel
GAPIC targets.

[vam-google]

LGTM if that works on the whole googleapis repo

[codecov]

Codecov Report

Merging #3222 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3222      +/-   ##
============================================
- Coverage     87.13%   87.12%   -0.01%     
+ Complexity     6078     6077       -1     
============================================
  Files           494      494              
  Lines         24052    24052              
  Branches       2613     2613              
============================================
- Hits          20957    20956       -1     
  Misses         2234     2234              
- Partials        861      862       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/google/api/codegen/discovery/Schema.java	84.34% <0.00%> (-0.51%)	42.00% <0.00%> (-1.00%)

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[noahdietz]

bazel build //... passes in googleapis with this change (and with no uses of proto3_optional in any of the protos)