feat: support for ipk packages

[schmidtw]

Implements #507.

• Adds ipk support for keywords used by OpenWRT and Yocto.
• MD5sum is explicitly excluded due to insecurity.
• SHA256Sum excluded due packages not being individually signed, instead, the feed of packages is checksummed and signed externally.
• Adds code to nfpm package to automatically enumerate the supported packaging types where possible.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 64 lines in your changes are missing coverage. Please review.

Project coverage is 72.65%. Comparing base (409b516) to head (2210695). Report is 4 commits behind head on main.

Files	Patch %	Lines
ipk/ipk.go	86.88%	13 Missing and 11 partials :warning:
ipk/tar.go	58.49%	11 Missing and 11 partials :warning:
nfpm.go	15.38%	10 Missing and 1 partial :warning:
internal/cmd/package.go	0.00%	4 Missing :warning:
internal/cmd/root.go	0.00%	2 Missing :warning:
cmd/nfpm/main.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #818 +/- ## ========================================== + Coverage 72.19% 72.65% +0.46% ========================================== Files 20 22 +2 Lines 2237 2487 +250 ========================================== + Hits 1615 1807 +192 - Misses 438 473 +35 - Partials 184 207 +23 ```

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[schmidtw]

@caarlos0 any feedback?

[caarlos0]

sorry, haven't had the time to look into it yet... will probably review over the weekend.

at first glance, it does look good though!

also, thanks for the PR 🙏🏻

[caarlos0]

it looks awesome, thanks!

and sorry for the delay reviewing it!