[FEA] set reader numThreads dynamically by default

[revans2]

Is your feature request related to a problem? Please describe.
In an effort to reduce the number of configs that a user must set we should try and have a better default for spark.rapids.sql.multiThreadedRead.numThreads. The auto-tuner currently sets this to max(80, executor.core) for CSPs and max(20, 2*executor.core) for on prem. We should know approximately the number of executor cores in the plugin, but we don't know always if we are in a CSP or on prem. We should run some tests and see if we can figure out a good default, beyond the 20 we have today.