labtunnel: Add tunnel operation for btpeers

Creating tunnels to bluetooth peers can be cumbersome to do manually,
so this change adds support for a new "btpeers" tunnel operation to
do to automatically like it does other tunnels. The README and usage
has been updated to describe this new feature as well.

BUG=None
TEST=Manually tested running the new tunnel operation and all of its
new related options.

Change-Id: I8d7ae6c7c8b78827cbb9c8470b078f5dafb6cab8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/3692167
Commit-Queue: Jared Bennett <jaredbennett@google.com>
Auto-Submit: Jared Bennett <jaredbennett@google.com>
Reviewed-by: Shijin Abraham <shijinabraham@google.com>
Tested-by: Jared Bennett <jaredbennett@google.com>
3 files changed