[GOBBLIN-960]Resolving multiple templates in top-level

[autumnust]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• https://issues.apache.org/jira/browse/GOBBLIN-960

Description

• Here are some details about my PR, including screenshots (if applicable):
  It contains some fixes on access-modifier of Avro2ORC while reading through code, but the major part is to enabled gobblin.templates.uri to be able to accept multiple templates.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[autumnust]

@jack-moseley @sv2000 Can you take a look once you have time? Thanks

[codecov-io]

Codecov Report

Merging #2809 into master will decrease coverage by 0.94%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master    #2809      +/-   ##
============================================
- Coverage     45.47%   44.52%   -0.95%     
+ Complexity     8909     8749     -160     
============================================
  Files          1899     1899              
  Lines         71098    71132      +34     
  Branches       7833     7846      +13     
============================================
- Hits          32330    31671     -659     
- Misses        35786    36522     +736     
+ Partials       2982     2939      -43

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/apache/gobblin/util/AvroFlattener.java	71.33% <ø> (ø)	33 <0> (ø)	⬇️
...he/gobblin/runtime/template/StaticJobTemplate.java	82.5% <100%> (+3.08%)	14 <1> (+1)	⬆️
...rg/apache/gobblin/runtime/AbstractJobLauncher.java	58.99% <100%> (-0.15%)	34 <5> (+1)
...n/java/org/apache/gobblin/runtime/api/JobSpec.java	80.13% <80%> (-0.01%)	17 <0> (ø)
...obblin/runtime/template/InheritingJobTemplate.java	90.36% <81.39%> (+0.22%)	29 <3> (+9)	⬆️
...re/filesystem/FsDatasetStateStoreEntryManager.java	0% <0%> (-100%)	0% <0%> (-3%)
...askStateCollectorServiceHiveRegHandlerFactory.java	0% <0%> (-100%)	0% <0%> (-2%)
...gobblin/runtime/mapreduce/GobblinOutputFormat.java	0% <0%> (-100%)	0% <0%> (-2%)
...in/runtime/mapreduce/CustomizedProgresserBase.java	0% <0%> (-83.34%)	0% <0%> (-1%)
...rg/apache/gobblin/runtime/ZkDatasetStateStore.java	0% <0%> (-80.77%)	0% <0%> (-7%)
... and 43 more

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[jack-moseley]

Maybe add some javadoc for resolveTemplates method since this logic is a bit confusing

[autumnust]

Added

[sv2000]

+1. LGTM.