middleware for content type and accept headers (#14075)

* middleware for content type

* adding accept middleware too and tests

* Update beacon-chain/rpc/endpoints.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update beacon-chain/rpc/endpoints.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update beacon-chain/rpc/endpoints.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update beacon-chain/rpc/endpoints.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* including radek's review

---------

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

api/server/middleware/util_test.go

@@ -0,0 +1,21 @@
+package middleware
+
+import (
+	"testing"
+
+	"github.com/prysmaticlabs/prysm/v5/testing/assert"
+	"github.com/prysmaticlabs/prysm/v5/testing/require"
+)
+
+func TestNormalizeQueryValues(t *testing.T) {
+	input := make(map[string][]string)
+	input["key"] = []string{"value1", "value2,value3,value4", "value5"}
+
+	NormalizeQueryValues(input)
+	require.Equal(t, 5, len(input["key"]))
+	assert.Equal(t, "value1", input["key"][0])
+	assert.Equal(t, "value2", input["key"][1])
+	assert.Equal(t, "value3", input["key"][2])
+	assert.Equal(t, "value4", input["key"][3])
+	assert.Equal(t, "value5", input["key"][4])
+}