[Request] Specify own DNS server

**Describe the request**
I'd love to be able to specify my own list of DNS servers, instead of using those provided by the system.

**How to Reproduce**
Not applicable. According to `DnsServerMapper.java`'s function `getNetworkDnsServers()` DNS servers are obtained from the active network's settings.

**Expected behavior**
I am able to specify a list of IP addresses of my preferred DNS servers. When they are specified, they are used instead of the system ones.

**Device**
- AdAway version: 5.8.0
- Android version: 11
- Root version: Not rooted, using the VPN functionality.

**Additional context**
- Older versions of Blokada allowed users to check/toggle DNS servers that would be used instead of the default ones.
- Forcing custom DNS servers may cause problems on some networks. This could be solved by adding a list of networks where the custom DNS servers should not be used. On the other hand, there can be just a note that setting own DNS server may cause issues and that the simplest solution would be to termporarily disable this feature while the user is connected to the network.