KDEV-784: Create function should support also a single item

[b-stolyarov]

Description

The create function should support also a single item.

Changes

Added create(entity: T, callback: KinveyClientCallback<T>) function to DataStore class.

Tests

Instrumented.

[codecov-commenter]

Codecov Report

Merging #285 into master will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #285      +/-   ##
============================================
+ Coverage     62.48%   62.76%   +0.27%     
- Complexity      609      622      +13     
============================================
  Files            43       44       +1     
  Lines          2844     2860      +16     
  Branches        841      845       +4     
============================================
+ Hits           1777     1795      +18     
+ Misses          534      530       -4     
- Partials        533      535       +2     

Impacted Files	Coverage Δ	Complexity Δ
...ava/com/kinvey/android/async/AsyncCreateRequest.kt	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...rc/main/java/com/kinvey/android/store/DataStore.kt	68.05% <100.00%> (-0.24%)	48.00 <1.00> (+2.00)	⬇️
...c/main/java/com/kinvey/android/cache/RealmCache.kt	78.06% <0.00%> (+0.05%)	154.00% <0.00%> (+1.00%)
.../com/kinvey/android/async/AsyncBatchPushRequest.kt	47.05% <0.00%> (+3.92%)	24.00% <0.00%> (+8.00%)
.../java/com/kinvey/android/AndroidCredentialStore.kt	58.00% <0.00%> (+4.00%)	9.00% <0.00%> (ø%)

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[alexanderfilipov]

LGTM as a whole.