Added config DNS discovery Resolves #4321

[OlegJakushkin]

Resolves #4321

Changes:

• Added configuration option for Dns Discovery similar to what geth has.

Note: we have no verification of a DNS list signer! Following EIP-1459 "public key must be known to the client to verify the list." Thus there shall be a list of public keys that a client allows, and we shall check against it. Geth also seems not to check it.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests??

• Yes
• No

Tested only on public DNS nodes.

[smartprogrammer93]

Other than the description of the config, all looks good.