feat: introduce bom artifact and make this a multi-module project

[chingor13]

Prepares the repository for adding gapic clients.

[codecov]

Codecov Report

Merging #419 (6de2775) into master (aa484e0) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #419   +/-   ##
=========================================
  Coverage     82.19%   82.19%           
  Complexity      248      248           
=========================================
  Files            13       13           
  Lines          1331     1331           
  Branches        141      141           
=========================================
  Hits           1094     1094           
  Misses          167      167           
  Partials         70       70           

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/resourcemanager/ConstraintInfo.java	82.85% <ø> (ø)	15.00 <0.00> (?)
.../java/com/google/cloud/resourcemanager/Option.java	86.66% <ø> (ø)	8.00 <0.00> (?)
...om/google/cloud/resourcemanager/OrgPolicyInfo.java	82.85% <ø> (ø)	15.00 <0.00> (?)
...google/cloud/resourcemanager/PolicyMarshaller.java	100.00% <ø> (ø)	6.00 <0.00> (?)
...java/com/google/cloud/resourcemanager/Project.java	96.15% <ø> (ø)	16.00 <0.00> (?)
.../com/google/cloud/resourcemanager/ProjectInfo.java	92.17% <ø> (ø)	27.00 <0.00> (?)
.../google/cloud/resourcemanager/ResourceManager.java	81.48% <ø> (ø)	0.00 <0.00> (?)
...loud/resourcemanager/ResourceManagerException.java	100.00% <ø> (ø)	5.00 <0.00> (?)
...gle/cloud/resourcemanager/ResourceManagerImpl.java	87.56% <ø> (ø)	24.00 <0.00> (?)
.../cloud/resourcemanager/ResourceManagerOptions.java	85.71% <ø> (ø)	11.00 <0.00> (?)
... and 3 more

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