encoding: Add the ZlibCompress function

[razonyang]

The PR adds zlib compression support to the template, which is useful for generating Kroki diagrams without requiring remote APIs, reducing build time and avoiding network issues.

Template usage:

{{ $diagram := `blockdiag {
  Kroki -> generates -> "Block diagrams";
  Kroki -> is -> "very easy!";

  Kroki [color = "greenyellow"];
  "Block diagrams" [color = "pink"];
  "very easy!" [color = "orange"];
}`
}}
{{- $encoded := encoding.ZlibCompress $diagram | encoding.Base64Encode }}
{{- $encoded = replace $encoded "+" "-" }}
{{- $encoded = replace $encoded "/" "_" }}
{{- $encoded = replace $encoded "=" "" }}
<img src="https://kroki.io/blockdiag/svg/{{ $encoded }}">

If this feature is not going to be accepted, please feel free to close it.

[bep]

See https://github.com/gohugoio/hugo/blob/master/CONTRIBUTING.md#code-contribution

Any non-trivial code change needs to update an open issue. A non-trivial code change without an issue reference with one of the labels bug or enhancement will not be merged.

Create a proposal first.

[razonyang]

Sorry I missed that, issue created #12868.