Adding Avro support for backup & restore

[codecov[bot]]

Codecov Report

Merging #105 into dev will increase coverage by 3.91%. The diff coverage is 57%.

@@             Coverage Diff              @@
##                dev     #105      +/-   ##
============================================
+ Coverage     48.44%   52.36%   +3.91%     
- Complexity      107      229     +122     
============================================
  Files            15       21       +6     
  Lines          1094     1694     +600     
  Branches        136      276     +140     
============================================
+ Hits            530      887     +357     
- Misses          521      705     +184     
- Partials         43      102      +59

Impacted Files	Coverage Δ	Complexity Δ
.../cloud/pontem/AvroCloudSpannerDatabaseRestore.java	0% <0%> (ø)	0 <0> (?)
...e/cloud/pontem/AvroCloudSpannerDatabaseBackup.java	50.92% <50.92%> (ø)	3 <3> (?)
...pontem/FormatGenericRecordAsSpannerMutationFn.java	62.06% <62.06%> (ø)	36 <36> (?)
...java/com/google/cloud/pontem/TableInformation.java	66.66% <66.66%> (ø)	9 <9> (?)
...le/cloud/pontem/FormatStructAsGenericRecordFn.java	80.41% <80.41%> (ø)	37 <37> (?)
...rc/main/java/com/google/cloud/pontem/AvroUtil.java	83.63% <83.63%> (ø)	22 <22> (?)
...main/java/com/google/cloud/pontem/SpannerUtil.java	46.05% <87.5%> (+14.8%)	29 <13> (+13)	:arrow_up:
... and 4 more