init add of Xamarin rules

[mike-hunhoff]

Adding new rules based on #706 . This PR may be impacted by the discussions in #818 regarding mobile ATT&CK techniques. I wanted to open a PR for discussion before diving too far into creating new rules.

Questions:

• Should Android example files be placed in separate tests folder?

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[williballenthin]

so cool to see

[mr-tz]

I think the testfiles should be separate.

Similarly, do we want to keep the rules files separate? Some rules contain various formats/OS so that's a little messy.
There's pro and cons, so maybe we better open up a separate discussion?

[mr-tz]

Collecting my thoughts here but happy to spin off separate issues/discussions.

• I like the in .NET on Android naming
• so far rules are inconsistent requiring format/os
  • this doesn't seem to be a problem though
• important are:
  • rule readability and
  • organization
    • 1. duplicate all directories under android parent?
    • 2. only update namespaces to start with android/ root <-- my preferred option
• testfiles should go into android directory

related discussion: mandiant/capa#701 (Rule organization for multiple file types PE and ELF)