- add sane default dns provider
- add http timeout
- add comment for ssl verification
- add proper logging
- remove option to add custom resolver (not used/needed)
- add test

Test run if the first two dns servers are dropped by firewall :

```
pea@peabox:bootstrap go test -count=1 -v ./...
=== RUN   TestDoh
10:26AM DBG Selected DoH provider dnsServer=208.67.222.222
10:26AM WRN Could not resolve host with DNS over HTTPs error="Post \"https://208.67.222.222/dns-query\": context deadline exceeded (Client.Timeout exceeded while awaiting headers)" domain=heise.de resolver=208.67.222.222
10:26AM DBG Selected DoH provider dnsServer=9.9.9.9
10:26AM WRN Could not resolve host with DNS over HTTPs error="Post \"https://9.9.9.9/dns-query\": context deadline exceeded (Client.Timeout exceeded while awaiting headers)" domain=heise.de resolver=9.9.9.9
10:26AM DBG Selected DoH provider dnsServer=dns.mullvad.net
10:26AM INF Got dns ip=193.99.144.80
--- PASS: TestDoh (30.16s)
PASS
ok      0xacab.org/leap/bitmask-core/pkg/bootstrap      30.172s
```