Implement decryption for cloud blobstore download blob

[ankagrawal]

for cases where the blob was encrypted by cloud blobstore before uploading

[lightningrob]

Overall looks good, few minor things.

[codecov-io]

Codecov Report

Merging #1240 into master will decrease coverage by 16.62%.
The diff coverage is 90.47%.

@@              Coverage Diff              @@
##             master    #1240       +/-   ##
=============================================
- Coverage     88.92%   72.29%   -16.63%     
- Complexity       60     6129     +6069     
=============================================
  Files             6      440      +434     
  Lines           352    35251    +34899     
  Branches         37     4481     +4444     
=============================================
+ Hits            313    25484    +25171     
- Misses           29     8603     +8574     
- Partials         10     1164     +1154

Impacted Files	Coverage Δ	Complexity Δ
...java/com.github.ambry.cloud/CloudBlobMetadata.java	95.71% <100%> (ø)	33 <4> (?)
...va/com.github.ambry.cloud/CloudMessageReadSet.java	50% <50%> (ø)	3 <0> (?)
...in/java/com.github.ambry.cloud/CloudBlobStore.java	82.77% <91.66%> (ø)	66 <2> (?)
...om/github/ambry/account/AccountServiceMetrics.java	100% <0%> (ø)	1% <0%> (ø)	⬇️
...ava/com.github.ambry/config/PerformanceConfig.java	100% <0%> (ø)	4% <0%> (?)
...main/java/com.github.ambry.cloud/CloudReplica.java	73.91% <0%> (ø)	8% <0%> (?)
...m.github.ambry.replication/ReplicationMetrics.java	94.92% <0%> (ø)	43% <0%> (?)
...ava/com.github.ambry/frontend/SecurityService.java	100% <0%> (ø)	4% <0%> (?)
...b.ambry.cloud/CloudBlobCryptoAgentFactoryImpl.java	81.81% <0%> (ø)	2% <0%> (?)
...ain/java/com.github.ambry/frontend/Operations.java	0% <0%> (ø)	0% <0%> (?)
... and 429 more

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[lightningrob]

Approved after addressing one comment.