Add get_owner() msg to *DNS* contract

[agryaznov]

As domain name is a tradable entity, there should be a way to get its current owner in a UI-friendly way, to ensure that the seller (which could be a contract) owns the domain being sold.

For that, get_owner() message is proposed.

Case study:

Let say a domain name is being put on a candle auction. Before trading starts, the domain owner should transfer the domain to the auction contract instance. Bidders need some method to check that the auction contract has the ownership of the domain in question. For that, they would use this get_owner() message.

[codecov-commenter]

Codecov Report

Merging #1118 (49a6775) into master (7721622) will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1118      +/-   ##
==========================================
+ Coverage   75.34%   75.67%   +0.32%     
==========================================
  Files         252      252              
  Lines        9395     9395              
==========================================
+ Hits         7079     7110      +31     
+ Misses       2316     2285      -31     

Impacted Files	Coverage Δ
...ates/storage/src/collections/hashmap/fuzz_tests.rs	95.74% <0.00%> (+2.12%)	⬆️
crates/engine/src/ext.rs	72.41% <0.00%> (+6.89%)	⬆️
crates/engine/src/test_api.rs	94.04% <0.00%> (+20.23%)	⬆️
crates/primitives/src/key_ptr.rs	100.00% <0.00%> (+25.00%)	⬆️

---

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[paritytech-cicd-pr]

🦑 📈 ink! Example Contracts ‒ Changes Report 📉 🦑

These are the results when building the examples/* contracts from this branch with cargo-contract 0.17.0-06c5598 and comparing them to ink! master:

	Δ Optimized Size	Δ Used Gas	Total Optimized Size	Total Used Gas
accumulator			1.44 K
adder			2.94 K
contract-terminate			1.31 K	200_778
contract-transfer			7.98 K	1_399
delegator			10.06 K	9_510
dns	+0.31 K	+327	10.78 K	14_615
erc1155			27.50 K	47_811
erc20			10.22 K	9_757
erc721			13.70 K	34_702
flipper			1.76 K	347
incrementer			1.64 K	340
multisig			28.56 K	54_896
proxy			3.81 K	2_629
rand-extension			5.12 K	6_654
subber			2.96 K
trait-erc20			10.52 K	10_644
trait-flipper			1.53 K	342
trait-incrementer			1.62 K	680

Link to the run | Last update: Mon Jan 31 10:17:11 CET 2022

[cmichi]

LGTM, thank you!