[infra-netif] select infrastructure link among multiple candidates (#1353)

This commit implements a general Infrastructure Link Selection that
allows otbr-agent to select one network interface among multiple
network interface candidates (specified by multiple `-B` arguments) as
the infrastructure link.

The Infrastructure Link Selection uses following rules:
- The network link in the most usable state is selected
  - Prefer `up and running` to `up`
  - Prefer `up` to `down`
  - Prefer `down` to `invalid`
- Once an interface is selected, it's preferred if either is true:
- The interface is still `up and running`
  - No other interface is `up and running`
  - The interface has been `up and running` within last 10 seconds

`otbr-agent` uses Infrastructure Link Selection to select the correct
infrastructure link:
- Run Infrastructure Link Selection when started to select the
  infrastructure link
- Run Infrastructure Link Selection when network interface states
  change is detected. otbr-agent would quit if a different
  infrastructure link is selected, expecting the system to restart it
  to use the new infrastructure link.

Note:
- The Infrastructure Link Selection mechanism is effectively disabled
  if less than two infrastructure link names are specified.
- The current implementation only works for Linux platforms.

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