feat(fqdn): Deduplicate and sort ExecTemplate output. Add functions

[ivankatliarchuk]

What does it do ?

• Make sure FQDN templating only output unique values, to reduce deduplication efforts downstream
• Adds a fromJson template function that decodes a JSON string into a Go value (map, slice, etc.) - Enables FQDN templates to work with structured data stored as JSON strings in ConfigMap data fields. For example, a ConfigMap can hold a JSON array of DNS entries, and templates can range over them to generate hostnames:
  {{ range $entry := (index .Data "entries" | fromJson) }}{{ index $entry "dns" }},{{ end }}
• Adds a hasKey template function that checks whether a key exists in a map[string]string. There is no other way to find where label is empty value is set on resourse - Go templates' built-in index returns the zero value ("") for missing keys, which is indistinguishable from keys with empty values. Kubernetes uses empty-value labels
  as markers (e.g. service.kubernetes.io/headless: ""), so templates need explicit key existence checking to conditionally generate hostnames based on label presence

Motivation

Templates can produce duplicate hostnames (e.g. multiple template expressions resolving to the same FQDN, or combineFQDNAnnotation merging overlapping results). Duplicates cause unnecessary DNS record creation attempts and then deduplications. Sorting ensures deterministic output, preventing spurious diff aka ExecTemplate returns ["b.com", "a.com"] one reconciliation and ["a.com", "b.com"] the next

slicing PR #6172

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 79.174%

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Totals
Change from base Build 21559764040:	0.02%
Covered Lines:	16051
Relevant Lines:	20273

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💛 - Coveralls

[ivankatliarchuk]

/retitle feat(fqdn): Deduplicate and sort ExecTemplate output. Add functions