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 #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!

[EdSchouten]

In #192 we rewrote rules_jsonnet to use toolchains. This means that it should be easier to add support for alternative Sonnet implementations. I'm going to close this PR due to inactivity, and also because it requires more work to apply cleanly. Feel free to submit another PR if you're still interested in adding support for sjsonnet.