Refactor interface availability code

[bugadani]

This PR builds on top of #8 and enables bitbang-dap to set the correct capabilities based on how many pins the user provides to the constructor (in theory, unimplemented atm.).

The SWO changes allow users to:

• Omit a bunch of boilerplate if they don't need SWO support
• Correctly set the streaming trace capability. This is a best guess on my part of what this bit means, based on "support for streaming SWO trace is provided via an additional USB endpoint".
• I'm trying to come up with a way to dynamically make SWO available if there are pins available (e.g. if the probe has 4 pins, allow SWD+SWO or JTAG+no-swo). I may end up not actually doing this, but this PR takes a step in that direction.