feat: add google-cloud-core-bom artifact

[chingor13]

According to JLBP-15, all multi-module projects should produce a bom to keep versions in sync.

The google-cloud-bom can import this bom, rather than specifying the versions.

[codecov]

Codecov Report

Merging #13 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #13      +/-   ##
============================================
- Coverage     66.46%   66.43%   -0.04%     
- Complexity      362      367       +5     
============================================
  Files            34       34              
  Lines          1858     1865       +7     
  Branches        234      236       +2     
============================================
+ Hits           1235     1239       +4     
- Misses          521      524       +3     
  Partials        102      102

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/com/google/cloud/RetryHelper.java	50% <0%> (-3.85%)	2% <0%> (ø)
...src/main/java/com/google/cloud/ServiceOptions.java	41.73% <0%> (+0.06%)	30% <0%> (+1%)	⬆️
...a/com/google/cloud/testing/BaseEmulatorHelper.java	31.42% <0%> (+0.21%)	5% <0%> (ø)	⬇️
.../com/google/cloud/MonitoredResourceDescriptor.java	84.03% <0%> (+0.27%)	24% <0%> (+4%)	⬆️

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[elharo]

should this version be synced with the rest of the repo?

[chingor13]

Fixed the version of the bom. The parent project is an external parent that contains the shared build/testing/release configuration across the split repositories.