inject-utils: Cross-build for 2.13

[chrisbenincasa]

Depends on #530

Problem

injectUtils was not cross-built for Scala 2.13.

Solution

Add SBT setting to enable 2.13 cross-build for injectUtils

NOTE This has some of the first usages of the Scala collection compat library. https://github.com/twitter/finagle/issues/818 will probably be relevant here due to some of the changes.

Result

injectUtils cross-built for Scala 2.13.

[codecov[bot]]

Codecov Report

Merging #532 into develop will decrease coverage by 0.01%. The diff coverage is 92.30%.

@@             Coverage Diff             @@
##           develop     #532      +/-   ##
===========================================
- Coverage    90.75%   90.73%   -0.02%     
===========================================
  Files          267      267              
  Lines         4779     4780       +1     
  Branches       301      297       -4     
===========================================
  Hits          4337     4337              
- Misses         442      443       +1     

Impacted Files	Coverage Δ
...ain/scala/com/twitter/inject/conversions/seq.scala	76.92% <50.00%> (-6.42%)	:arrow_down:
...cala/com/twitter/inject/conversions/iterable.scala	100.00% <100.00%> (ø)
...ain/scala/com/twitter/inject/conversions/map.scala	100.00% <100.00%> (ø)
.../scala/com/twitter/inject/conversions/option.scala	100.00% <100.00%> (ø)
...n/scala/com/twitter/inject/conversions/tuple.scala	94.11% <100.00%> (ø)

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[yufangong]

Merged at https://github.com/twitter/finatra/commit/94c43c54c678c7d955f82495c797bcec24925c3b. TY!