[BigQueryWorkloadTester] Adding BigQuery Credentials Manager and test

[ldanielmadariaga]

[BigQueryWorkloadTester] Adding BigQuery Credentials Manager and test

[codecov[bot]]

Codecov Report

Merging #193 into dev will decrease coverage by 73.18%. The diff coverage is 52.63%.

@@             Coverage Diff              @@
##                dev    #193       +/-   ##
============================================
- Coverage     76.65%   3.47%   -73.19%     
+ Complexity      344       7      -337     
============================================
  Files            34      15       -19     
  Lines          3256     288     -2968     
  Branches        266      10      -256     
============================================
- Hits           2496      10     -2486     
+ Misses          669     277      -392     
+ Partials         91       1       -90

Impacted Files	Coverage Δ	Complexity Δ
...e/cloud/pontem/auth/BigQueryCredentialManager.java	44.44% <44.44%> (ø)	3 <3> (?)
...oud/pontem/auth/BigQueryCredentialManagerTest.java	60% <60%> (ø)	4 <4> (?)
.../cloud/pontem/BaseCloudSpannerDatabaseRestore.java
.../cloud/pontem/AvroCloudSpannerDatabaseRestore.java
...loud/pontem/FormatStructAsGenericRecordFnTest.java
...oud/pontem/AvroCloudSpannerDatabaseBackupTest.java
...ntem/CloudSpannerDatabaseBackupIntegrityCheck.java
...tem/FormatSpannerTablesListStructAsTextFnTest.java
...java/com/google/cloud/pontem/TableInformation.java
...CloudSpannerDatabaseRestoreIntegrityCheckTest.java
... and 41 more