[LIVY-891] Expose the sessionID via the client APIs [WIP]

[haripriya-b]

https://issues.apache.org/jira/browse/LIVY-891

What changes were proposed in this pull request?

Currently getSessionID() is exposed in HttpClient only for testing purposes. Since the internally created session ID is useful to doing operations such as re-connecting to the session, killing the session etc., it will be useful to expose it as a public api in HttpClient and LivyClient. (Include a link to the associated JIRA and make sure to add a link to this pr on the JIRA as well)

How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests) (If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review https://livy.incubator.apache.org/community/ before opening a pull request.

[codecov-commenter]

Codecov Report

Merging #348 (2369bdc) into master (4d8a912) will decrease coverage by 0.20%. The diff coverage is 76.92%.

@@             Coverage Diff              @@
##             master     #348      +/-   ##
============================================
- Coverage     68.50%   68.29%   -0.21%     
- Complexity      841      843       +2     
============================================
  Files           103      103              
  Lines          5940     5968      +28     
  Branches        898      907       +9     
============================================
+ Hits           4069     4076       +7     
- Misses         1312     1331      +19     
- Partials        559      561       +2     

Impacted Files	Coverage Δ
...c/src/main/java/org/apache/livy/rsc/RSCClient.java	72.61% <0.00%> (-0.88%)	:arrow_down:
...n/java/org/apache/livy/client/http/HttpClient.java	85.71% <100.00%> (+0.23%)	:arrow_up:
...c/main/scala/org/apache/livy/repl/ReplDriver.scala	27.27% <0.00%> (-8.63%)	:arrow_down:
...ain/scala/org/apache/livy/utils/SparkYarnApp.scala	72.61% <0.00%> (-2.39%)	:arrow_down:
...ain/java/org/apache/livy/rsc/driver/RSCDriver.java	79.66% <0.00%> (-1.17%)	:arrow_down:
...scala/org/apache/livy/repl/PythonInterpreter.scala	44.17% <0.00%> (ø)
...rg/apache/livy/repl/AbstractSparkInterpreter.scala	57.14% <0.00%> (ø)
...main/scala/org/apache/livy/server/LivyServer.scala	32.14% <0.00%> (+0.44%)	:arrow_up:
...cala/org/apache/livy/scalaapi/ScalaJobHandle.scala	65.11% <0.00%> (+2.32%)	:arrow_up:
...ala/org/apache/livy/utils/LineBufferedStream.scala	82.92% <0.00%> (+2.43%)	:arrow_up:
... and 1 more

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more