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>