feat: Multiple fixes to make continuous mode work

[jiangmichaellll]

This brings the continuous mode working with non-uber jar solution. Uber jar still won't work.

Mainly:

1. Lower scala version to 2.11 since it's what's provided in spark 2.4.7 in classpath
2. Fixes the conversion from java types into spark types
3. Added maven shade plugin for uber jar generation

[codecov]

Codecov Report

Merging #432 (ffd15b3) into master (34d8d02) will increase coverage by 0.07%.
The diff coverage is 83.87%.

@@             Coverage Diff              @@
##             master     #432      +/-   ##
============================================
+ Coverage     71.15%   71.22%   +0.07%     
- Complexity      902      905       +3     
============================================
  Files           167      167              
  Lines          4770     4789      +19     
  Branches        240      242       +2     
============================================
+ Hits           3394     3411      +17     
- Misses         1238     1240       +2     
  Partials        138      138              

Impacted Files	Coverage Δ	Complexity Δ
...cloud/pubsublite/spark/PslCredentialsProvider.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...m/google/cloud/pubsublite/spark/PslDataSource.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...m/google/cloud/pubsublite/spark/PslSparkUtils.java	93.33% <100.00%> (+2.63%)	13.00 <5.00> (+4.00)

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