opa eval: support rego validation using input JSON schema (#3060)

New support to input a JSON schema file via "opa eval --schema" that
helps improve static type checking and returns precise error reports as
you develop Rego code.

Also, uses modified version of https://github.com/xeipuuv/gojsonschema
from internal/ to help set Rego types, using gojsonschema's Compiler
results. See README docs for more details on the feature.

Co-authored-by: Mandana Vaziri <mvaziri@us.ibm.com>
Co-authored-by: Ansu Varghese <avarghese@us.ibm.com>
Signed-off-by: Ansu Varghese <avarghese@us.ibm.com>