feat: expose timeStorageClassUpdated property of blob's

[athakor]

Fixes #455

[codecov]

Codecov Report

Merging #456 into master will increase coverage by 0.10%.
The diff coverage is 92.85%.

@@             Coverage Diff              @@
##             master     #456      +/-   ##
============================================
+ Coverage     64.59%   64.69%   +0.10%     
+ Complexity      626      620       -6     
============================================
  Files            32       32              
  Lines          5236     5249      +13     
  Branches        507      510       +3     
============================================
+ Hits           3382     3396      +14     
+ Misses         1695     1694       -1     
  Partials        159      159              

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/com/google/cloud/storage/BlobInfo.java	88.34% <90.00%> (+0.04%)	94.00 <1.00> (+3.00)
...e/src/main/java/com/google/cloud/storage/Blob.java	83.70% <100.00%> (+1.32%)	30.00 <0.00> (ø)
...rc/main/java/com/google/cloud/storage/Storage.java	80.83% <100.00%> (+0.04%)	0.00 <0.00> (ø)

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[frankyn]

Follow the same pattern in custom time to raise an exception and implement this method to prevent breaking changes.

[athakor]

done

[athakor]

@frankyn PTAL