Use text/template instead of html/template for config pre-processing

Commit c95311d1 added support for multiple config formats in addition to
pre-processing using Go's built-in templating system. The
`html/template` package is equivalent to `text/template`, except that
the former automatically escapes characters for inclusion in HTML.

Configurations aren't plain text, but they're also certainly no HTML.
The difference between the packages is noticeable when using `printf
"%q"` for quoting of strings. An example from the included unittest:

* `html/template`: `key = "with space"`
* `text/template`: `key = "with space"`

Signed-off-by: Michael Hanselmann <public@hansmi.ch>

pkg/config/load.go

@@ -18,10 +18,10 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
-	"html/template"
 	"os"
 	"path/filepath"
 	"strings"
+	"text/template"
 
 	toml "github.com/pelletier/go-toml/v2"
 	"github.com/samber/lo"

pkg/config/load_test.go

@@ -112,6 +112,29 @@ func TestLoadServerConfigStrictMode(t *testing.T) {
 	}
 }
 
+func TestRenderWithTemplate(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		want    string
+	}{
+		{"toml", tomlServerContent, tomlServerContent},
+		{"yaml", yamlServerContent, yamlServerContent},
+		{"json", jsonServerContent, jsonServerContent},
+		{"template numeric", `key = {{ 123 }}`, "key = 123"},
+		{"template string", `key = {{ "xyz" }}`, "key = xyz"},
+		{"template quote", `key = {{ printf "%q" "with space" }}`, `key = "with space"`},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			require := require.New(t)
+			got, err := RenderWithTemplate([]byte(test.content), nil)
+			require.NoError(err)
+			require.EqualValues(test.want, string(got))
+		})
+	}
+}
+
 func TestCustomStructStrictMode(t *testing.T) {
 	require := require.New(t)