http mock server: Added glob template helper function

.changelog/165.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+http mock server: Added glob template helper function for iterating over multiple files.
+```

README.md

@@ -87,6 +87,7 @@ When using [Go templates](https://golang.org/pkg/text/template/) as part of the
 - `env KEY`: function that returns the KEY from environment.
 - `sum A B`: function that returns the sum of numbers A and B (only for integers).
 - `file PATH`: function that returns the contents of the file at PATH.
+- `glob PATTERN`: function that returns the names of all files matching glob PATTERN (see [filepath.Match](https://pkg.go.dev/path/filepath#Match) for syntax).
 - `.req_num`: variable containing the current request number, auto incremented after every request for the rule.
 - `.request.vars`: map containing the variables received in the request (both query and form).
 - `.request.url`: the url object. Can be used as per [the Go URL documentation.](https://golang.org/pkg/net/url/#URL)

internal/httpserver/config.go

@@ -8,6 +8,7 @@ import (
 	"encoding/json"
 	"errors"
 	"os"
+	"path/filepath"
 	"strings"
 	"text/template"
 
@@ -51,6 +52,7 @@ func (t *tpl) Unpack(in string) error {
 			"hostname":    hostname,
 			"sum":         sum,
 			"file":        file,
+			"glob":        filepath.Glob,
 			"minify_json": minify,
 		}).
 		Parse(in)