Cosmos key credential support

[kushagraThapar]

• Added Cosmos Key Credential support to CosmosDBFactory.
• Users can use single instance of CosmosKeyCredential to pass cosmos db account key to the DocumentDbConfig.
• To rotate keys, user just need to update the key in CosmosKeyCredential instance, and Cosmos DB SDK will start using new key for the future requests.

[codecov-io]

Codecov Report

Merging #408 into master will increase coverage by 0.14%. The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #408      +/-   ##
==========================================
+ Coverage   75.35%   75.49%   +0.14%     
==========================================
  Files          47       47              
  Lines        1351     1363      +12     
  Branches      152      154       +2     
==========================================
+ Hits         1018     1029      +11     
  Misses        254      254              
- Partials       79       80       +1

Impacted Files	Coverage Δ
.../spring/data/cosmosdb/config/DocumentDBConfig.java	96.55% <100%> (+1.31%)	:arrow_up:
...ft/azure/spring/data/cosmosdb/CosmosDbFactory.java	71.87% <87.5%> (+0.44%)	:arrow_up:

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