chore: Update versions to 0.3.0 / 0.3.0-SNAPSHOT

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

I have created branch-0.2 and started the release process for 0.2.0-rc1. The main branch now switches to 0.3.0 development.

What changes are included in this PR?

How are these changes tested?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.07%. Comparing base (9d8730d) to head (d5a4bb7).
Report is 10 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##               main     #868       +/-   ##
=============================================
- Coverage     55.16%   34.07%   -21.09%     
- Complexity      857      887       +30     
=============================================
  Files           109      112        +3     
  Lines         10542    42984    +32442     
  Branches       2010     9470     +7460     
=============================================
+ Hits           5815    14647     +8832     
- Misses         3714    25338    +21624     
- Partials       1013     2999     +1986     

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[viirya]

We also need to update Spark diffs:

[error]   not found: https://maven-central.storage-download.googleapis.com/maven2/org/apache/comet/comet-spark-spark4.0_2.13/0.2.0-SNAPSHOT/comet-spark-spark4.0_2.13-0.2.0-SNAPSHOT.pom
[error]   not found: https://repo1.maven.org/maven2/org/apache/comet/comet-spark-spark4.0_2.13/0.2.0-SNAPSHOT/comet-spark-spark4.0_2.13-0.2.0-SNAPSHOT.pom
[error]   not found: /root/.m2/repository/org/apache/comet/comet-spark-spark4.0_2.13/0.2.0-SNAPSHOT/comet-spark-spark4.0_2.13-0.2.0-SNAPSHOT.pom
[error]   not found: /root/.ivy2/localorg.apache.comet/comet-spark-spark4.0_2.13/0.2.0-SNAPSHOT/ivys/ivy.xm