Add basic support for radare2

[TrungNguyen1909]

Pwntools Pull Request

Thanks for contributing to Pwntools! Take a moment to look at CONTRIBUTING.md to make sure you're familiar with Pwntools development.

Please provide a high-level explanation of what this pull request is for.

Testing

Pull Requests that introduce new code should try to add doctests for that code. See TESTING.md for more information.

Target Branch

Depending on what the PR is for, it needs to target a different branch.

You can always change the branch after you create the PR if it's against the wrong branch.

Branch	Type of PR
dev	New features, and enhancements
dev	Documentation fixes and new tests
stable	Bug fixes that affect the current stable branch
beta	Bug fixes that affect the current beta branch, but not stable
dev	Bug fixes for code that has never been released

Hello,
Recently, I've made basic radare2 support which is in the r2.py in pwnlib
Most of them are copy from the gdb.
But I have managed to make attach(), version(), debug_shellcode() should works as expected.
I didn't have time to test a lot except attach() and version().
It's still need a lot of development but I don't have much time to continue recently.
Hopefully you or someone else can pick it up.
Btw, the comments aren't fully completed, It looks messy now but basically, it will not differ from gdb's one much.
Thanks

[zachriggle]

This looks like a copy-paste and find-replace of the GDB module, and doesn't actually add as much functionality as it appears.

I'd much rather add support for a new e.g. context.debugger which can be overridden to be e.g. r2 in the same way that context.terminal works.