Create "make install" equivalent

Open source projects are typically built in three phases:

1) Configuration (`./configure`), where the project build is customized to target specific featuresets
2) Building (`make`), where the project is actually built
3) Installation (`make install`), where the project is installed to the filesystem, or staged for future packaging using the [`DESTDIR` convention](https://www.gnu.org/prep/standards/html_node/DESTDIR.html).

Bazel combines 1) and 2) (configuring is done with build flags), and 3) doesn't really exist. Currently, the closest bazel equivalent to have bazel create a package representing the installable outputs, most often a .zip or a .tar. While effective, it is wasteful when packages become quite large.

Since Bazel builds have no way to interact directly with the filesystem outside of the buildroot, we need to be a little more creative. Let's create a `pkg_install` rule that creates a (python?) script that knows how to install artifacts outside of the bazel buildroot, for example:

```starlark
load("@rules_pkg//:install.bzl", "pkg_install")

# pkg_files, pkg_filegroups, etc

pkg_install(
name = "install",
srcs = [
# pkg_filegroups here
],
)
```

And then you install using:

```
bazel run //:install -- DESTDIR=your/destination/directory
```

Acceptance criteria:

- `pkg_install` (or its equivalent) is created with tests
- `pkg_install` supports staging to an arbitrary destination (`DESTDIR`)