Add `go_cross_binary` rule for cross-compilation. (#3261) * Add `go_cross_binary` rule for cross-compilation. - Adds a `go_cross_binary` rule that wraps a `go_binary` to generate a cross-compiled version of the binary. - Supports compiling for a different platform, and/or a different golang SDK version. - Adds docs for the new `go_cross_binary` rule. - Adds testing in `tests/core/cross` for the new `go_cross_binary` rule. Signed-off-by: James Bartlett <jamesbartlett@newrelic.com>