Add plugin interface for custom build steps

### What is the problem or limitation you are having?

We've had a number of requests to add customised steps into the build process:

* #436 - Running PySide "resource build" steps
* #1865 - Run obfuscation tooling over an app
* glyph/Encrust#2 - Compiling XIB files in a macOS project with ibtool

These are not necessarily processes that Briefcase should support out of the box, but there is clearly a need to support them. Briefcase should provide a plugin interface to allow libraries to add custom steps to the build.

### Describe the solution you'd like

A third party package should be able to register an entry point that will be invoked as part of the build command.

A post-build hook would be the bare minimum; it might make sense to expose a pre-build hook as well. Presumably these two entry points would be invoked before and after the `BuildCommand.build_app()` invocation as part of `_build_app()`.

The call to the plugin should be given the full BuildCommand as an argument, so that plugins can access tools, utility methods like `build_path()` etc.

### Describe alternatives you've considered

It might make sense to expose other entry points (pre/post-create, pre/post-run, pre/post-package etc).

As a convenience (stemming from the original feature request from #436), having a default plugin can run bare shell commands may be useful.

### Additional context

The interaction with Docker-based Linux builds may be a complication.

Start by tracing _build_app() and BuildCommand.build_app() to understand where build hooks could run, then inspect how Briefcase discovers command entry points. Consider the Docker-based Linux build path and determine how a third-party package receives the full BuildCommand. Done means the plugin interface and its invocation points are defined and covered by tests.

Written by the indexing model from the issue text.