Add DNS implementation

[soypat]

Still a WIP. Based heavily on golang.org/x/net/dns/dnsmessage module.

[codecov[bot]]

Codecov Report

Attention: 558 lines in your changes are missing coverage. Please review.

Comparison is base (289e517) 45.92% compared to head (70f01dd) 43.29%.

:exclamation: Current head 70f01dd differs from pull request most recent head 8765455. Consider uploading reports for the commit 8765455 to get more accurate results

Files	Patch %	Lines
eth/dns/types.go	53.82%	118 Missing and 33 partials :warning:
stacks/ntp_client.go	0.00%	104 Missing :warning:
stacks/dns_client.go	0.00%	97 Missing :warning:
eth/ntp/ntp.go	26.72%	82 Missing and 3 partials :warning:
stacks/dhcp_client.go	43.18%	46 Missing and 4 partials :warning:
eth/dns/stringers.go	0.00%	37 Missing :warning:
eth/dns/dns.go	58.57%	26 Missing and 3 partials :warning:
stacks/portstack.go	50.00%	2 Missing and 1 partial :warning:
valuesize.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #15 +/- ## ========================================== - Coverage 45.92% 43.29% -2.63% ========================================== Files 22 28 +6 Lines 3874 4679 +805 ========================================== + Hits 1779 2026 +247 - Misses 1873 2387 +514 - Partials 222 266 +44 ```

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[soypat]

Merged NTP branch since it had important UDP functionality needed by DNS branch (both work on top of UDP)

[soypat]

Merging branch as-is to use latest important changes to DHCP in CYW43439's examples