Add DynamicTaskConfig to store task configs in ZK

[rabashizade]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

Fixes #1207

Description

• Here are some details about my PR, including screenshots of any UI changes:

This PR adds DynamicTaskConfig class, which is a wrapper for ZNRecord, to store and access the configs for dynamically loaded tasks in ZK. It also adds the appropriate constants that refer to the config fields to TaskConstants.

Tests

• The following tests are written for this issue:

No tests were written, as this PR only adds a class to the project.

• The following is the result of the "mvn test" command on the appropriate module:

Since the PR doesn't modify any existing code, it's not going to cause any test failures. The following is the build result:

[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for Apache Helix 1.0.2-SNAPSHOT:
[INFO] 
[INFO] Apache Helix ....................................... SUCCESS [  2.163 s]
[INFO] Apache Helix :: Metrics Common ..................... SUCCESS [  4.236 s]
[INFO] Apache Helix :: Metadata Store Directory Common .... SUCCESS [  2.850 s]
[INFO] Apache Helix :: ZooKeeper API ...................... SUCCESS [  5.996 s]
[INFO] Apache Helix :: Helix Common ....................... SUCCESS [  3.138 s]
[INFO] Apache Helix :: Core ............................... SUCCESS [ 38.853 s]
[INFO] Apache Helix :: Admin Webapp ....................... SUCCESS [  5.833 s]
[INFO] Apache Helix :: Restful Interface .................. SUCCESS [ 10.435 s]
[INFO] Apache Helix :: Distributed Lock ................... SUCCESS [  1.894 s]
[INFO] Apache Helix :: HelixAgent ......................... SUCCESS [  2.897 s]
[INFO] Apache Helix :: Recipes ............................ SUCCESS [  0.044 s]
[INFO] Apache Helix :: Recipes :: Rabbitmq Consumer Group . SUCCESS [  1.426 s]
[INFO] Apache Helix :: Recipes :: Rsync Replicated File Store SUCCESS [  1.463 s]
[INFO] Apache Helix :: Recipes :: distributed lock manager  SUCCESS [  1.043 s]
[INFO] Apache Helix :: Recipes :: distributed task execution SUCCESS [  1.171 s]
[INFO] Apache Helix :: Recipes :: service discovery ....... SUCCESS [  1.046 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  01:25 min
[INFO] Finished at: 2020-08-04T09:50:49-07:00
[INFO] ------------------------------------------------------------------------

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation (Optional)

• In case of new functionality, my PR adds documentation in the following wiki page:

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[narendly]

Overall, let's go over this class and make sure we do not have methods (Getters and setters) that we don't/shouldn't need. Think about which fields should be immutable?

Also, let's put a focus on descriptive variable/method names as well.

[rabashizade]

This PR is ready to be merged, approved by @narendly
Final commit message:

Add DynamicTaskConfig to store task configs in ZK

Adds DynamicTaskConfig class, which is a wrapper for ZNRecord, to store
and access the configs for dynamically loaded tasks in ZK.

Also adds the appropriate constants to TaskConstants.