Add JSONata template evaluation via gnata

[Copilot]

Adds JSONata 2.x expression evaluation as a first-class option on the Template struct, backed by github.com/recolabs/gnata. Custom gomplate functions are exposed in the JSONata environment; gnata's native built-ins take precedence on any name collision.

Template struct

• New Jsonata string field (yaml:"jsonata", json:"jsonata")
• RunTemplateContext evaluates the Jsonata field after the existing CEL path
• IsEmpty, CacheKey, String, ShortString updated accordingly

jsonata.go — new file

• gnataBuiltinFunctions set guards against overriding native JSONata semantics
• getJSONataCustomFuncs() exposes gomplate-specific functions as gnata.CustomFunc wrappers, skipping any name in the builtin set:
  • Crypto: $sha1 / $sha224 / $sha256 / $sha384 / $sha512 / $sha512_224 / $sha512_256
  • Data: $toJSON / $fromJSON / $toYAML / $fromYAML
  • Query: $jq / $jmespath / $jsonpath
  • Regexp: $regexpFind / $regexpFindAll / $regexpMatch / $regexpReplace / $regexpReplaceLiteral / $regexpSplit
  • Format: $f — go-template format string (mirrors the CEL f() function)
• Shared customJSONataEnv initialized once at startup; expressions are cached per-expression string

Usage

out, err := gomplate.RunTemplate(env, gomplate.Template{
    Jsonata: `Account.Orders.$sum(Product.(Price * Quantity))`,
})

// Custom functions available in expressions:
out, err = gomplate.RunTemplate(nil, gomplate.Template{
    Jsonata: `$sha256("hello")`,
})
// → "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"

Tests (tests/jsonata_test.go)

New test suite mirroring the existing CEL tests: basic path/arithmetic, built-in string/array/math/object functions, all custom gomplate functions, and a precedence test confirming gnata native functions cannot be shadowed.

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