[kapt] allow options to be passed to kapt and javac annotation processing.

[restingbull]

Implements kapt_compiler_plugin, which allows the use of kt_plugin_cfg to pass option to annotation processors.

This is step one of moving kapt out of the builder, and enhancing the kt_compiler_plugin support.

Changes:

• resolve_cfg signature change (no one is using it, as it's limited and complicated) to return a list of providers. This allows a compiler plugin to export configurations in a clearer manner.
• Add annotation_processor_options to JavacOptions. This is not exposed on kt_javac_opts, as annotation_processor_options should always be associated with an annotation processor -- to avoid orphaned arguments being passed indefinitely. A rule could exploit this to make a mess, but that is on the author. I hope to avoid regretting this feature.
• modify base64 args encoding to optimize on single argument values. The whole protocol for this in kapt is a right mess.
• Fix javac annotation processing to include exported java plugins. Need better tests on this.
• And a heavy bunch of test code. Some refactoring could make it less verbose.

Example:

This will add the undocumented argument com.google.auto.value.OmitIdentifiers to AutoValue processing. :autovalue_no_identifiers can be added to kt_jvm_library.plugins to trigger annotation processing.

kt_plugin_cfg(
    name = "autovalue_no_identifiers",
    options = {
        "com.google.auto.value.OmitIdentifiers": ["true"], # declare the OmitIdentifiers option value.
    },
    plugin = "//kotlin/compiler:kapt", # targets the configuration to kapt
    deps = [
        ":autovalue", # adds the AutoValue annotation processor to javac and kotlinc kapt
    ],
)

towards: #1311

[restingbull]

fyi: @shs96c, @cheister