Support explicit API mode

[Bencodes]

Support for explicit API mode which was added in Kotlin 1.4.x.

https://github.com/Kotlin/KEEP/blob/master/proposals/explicit-api-mode.md

Example usage:

# Custom kotlinc options that can be provided at the target level 
# to override what is being set globally by `define_kt_toolchain`
kt_kotlinc_options(
    name = "kt_kotlinc_library_options",
    # ...
    x_explicit_api_mode = "strict",
)

kt_jvm_library(
    name = "foo_library",
    # ...
    kotlinc_opts = ":kt_kotlinc_library_options",
    srcs = glob(["*.kt"]),
    deps = [
        # ...
    ],
)

[jeffzoch]

@Bencodes is there anyway to only enable this on certain targets?

[Kernald]

Agreed, this option would make more sense on a per-target basis.

[Bencodes]

This can be configured at the target level using kotlinc_opts.

[Bencodes]

Added an example to the description