[FEA] Accelerate CI/CD by building and testing only the changed modules

[ttnghia]

The CI/CD time now is several hours, which is enormous. Sometimes, there are only very few changes in the code but they trigger the entire CI/CD pipeline to build and test for all unrelated modules. We can do much better by only building and testing the related ones for each PR.

For example, changes in the java folder would never affect the libcudf and cudf python modules thus we would never need to build and test them. Similarly, changes only in the python module should never trigger rebuilding and testing the libcudf as well as java module.

[vyasr]

Yes, this is a good idea. We're discussing this kind of change RAPIDS-wide in rapidsai/build-planning#5, so closing in favor that issue.