[VL] Support cudf.allowCpuFallback config for Velox cuDF backend

[ReemaAlzaid]

What changes are proposed in this pull request?

Adds spark.gluten.sql.columnar.backend.velox.cudf.allowCpuFallback
(default true) and passes it through to Velox's CudfConfig at backend
init, letting users control CPU fallback for the cuDF/Velox backend.

How was this patch tested?

by using gluten-it, with this command:

sbin/gluten-it.sh queries-compare \
  --local --preset=velox --benchmark-type=ds --error-on-memleak \
  --off-heap-size=10g -s=1 --threads=1 --iterations=1 \
  --decimal-as-double=true \
  --extra-conf=spark.gluten.sql.columnar.backend.velox.cudf.allowCpuFallback=false \
  --extra-conf=spark.gluten.sql.columnar.cudf=true

[ReemaAlzaid]

@marin-ma

[marin-ma]

@ReemaAlzaid To add a new user-exposed configuration, you also need to add an entry in VeloxConfig.scala, and run dev/gen-all-config-docs.sh to update the documentation.

[ReemaAlzaid]

Added them @marin-ma thanks

[marin-ma]

Thanks @ReemaAlzaid

[marin-ma]

This is not needed.