Chore: Query endpoint refactor (#41637)

Get rid of using legacydata.RequestHandler in HTTPServer, /api/tsdb/query and pkg/expr 
with the goal of deprecating /api/tsdb/query and remove it completely eventually. This is 
the first step of cleaning up the HTTP API query endpoint.

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
Co-authored-by: Alexander Emelin <frvzmb@gmail.com>

pkg/util/strings.go

@@ -6,6 +6,7 @@ import (
 	"regexp"
 	"strings"
 	"time"
+	"unicode"
 )
 
 // StringsFallback2 returns the first of two not empty strings.
@@ -108,3 +109,12 @@ func ToCamelCase(str string) string {
 
 	return strings.Join(finalParts, "")
 }
+
+func Capitalize(s string) string {
+	if len(s) == 0 {
+		return s
+	}
+	r := []rune(s)
+	r[0] = unicode.ToUpper(r[0])
+	return string(r)
+}