fix: Use thread context classloader for Iceberg class loading in CometScanRule

[chern]

Which issue does this PR close?

Closes #3737.

Rationale for this change

Similar change to #3738.

Comet uses Class.forName(className) to reflectively load Iceberg classes.
The single-argument Class.forName uses the classloader of the calling class, not the thread context classloader.

When the Comet JAR is placed in Spark's jars/ folder, it is loaded by the system classloader.
However, Iceberg classes may be bundled inside the user's application jar (as uber jar), which is loaded by Spark's application classloader .
Since parent classloaders cannot see classes loaded by child classloaders, Class.forName from Comet's code fails with ClassNotFoundException.

What changes are included in this PR?

• Replaces bare Class.forName() calls in CometScanRule with the IcebergReflection.loadClass() helper method.

How are these changes tested?

• Verified in a deployment where the Comet jar is in Spark's jars/ folder and Iceberg is bundled in the application's uber jar; ran a few SQL queries that involved reading from Iceberg.

[karuppayya]

@mbutrovich Do we need a sperate issue tracking this or reuse old?

[mbutrovich]

@mbutrovich Do we need a sperate issue tracking this or reuse old?

No preference.

[mbutrovich]

Thanks @chern!