Add DomainNameService example contract (#293)

* [examples] add DomainNameService example contract

* [examples] remove unnecessary constructor param

* [examples] fix clippy warning

* [examples] apply rustfmt

* [examples] update SCALE 1.1 -> 1.2

* [example] generally update the DNS contract example

* [examples] add Debug, PartialEq and Eq derives to Error

* [examples] add tests to DomainNameService example contract

* [lang] move default off-chain init before contract init

This is to prevent failures where the constructor of the contract already accesses certain off-chain fields.

* [examples] remove .cargo/config - no longer needed

* [examples] fix abi_gen impl for DNS contract

* fix typo

Co-Authored-By: Michael Müller <mich@elmueller.net>

Co-authored-by: Michael Müller <mich@elmueller.net>
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