WIP: Support sjsonnet as an alternative tool

[Qinusty]

This adds the option to add --define jsonnet_port=scala to bazel args to enable the use of the sjsonnet interpreter instead of the default go-jsonnet.

Progress 18th Jun: Currently there are some incompatibilities with the sjsonnet tool causing it to fail the test suite.

@examples//:extvar_env_test                                              FAILED in 0.8s
    ERROR   .external/examples/extvar_env_test (0.0s)
@examples//:extvar_files_test                                            FAILED in 0.8s
    ERROR   .external/examples/extvar_files_test (0.0s)
@examples//:extvar_files_test_filegroup                                  FAILED in 0.8s
    ERROR   .external/examples/extvar_files_test_filegroup (0.0s)
@examples//:extvar_stamp_test                                            FAILED in 0.8s
    ERROR   .external/examples/extvar_stamp_test (0.0s)
@examples//:invalid_test                                                 FAILED in 1.0s
    ERROR   .external/examples/invalid_test (0.0s)
@examples//:strings_test                                                 FAILED in 0.8s
    ERROR   .external/examples/strings_test (0.0s)
Test cases: finished with 10 passing and 6 failing out of 16 test cases

INFO: Build completed, 6 tests FAILED, 7 total actions

Addresses https://github.com/bazelbuild/rules_jsonnet/issues/132

[EdSchouten]

Considering that this PR is still WIP according to the title and the tests still fail, I have marked this PR as draft.

@Qinusty Feel free to mark it ready for review once it's ready. Thanks!