Add json schema and cli lint command (#342)

- Add json schema file
- Add tests to validate sample pipeline files
- Add new command `lint` to cli to test a directory or single file to use correct schema

Example:  `woodpecker-cli lint ./pipeline/schema/.woodpecker/`

---
close #275 
preparation for #276 


Co-authored-by: 6543 <6543@obermui.de>

shared/yml/yml.go

@@ -0,0 +1,71 @@
+package yml
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Source: https://github.com/icza/dyno/blob/f1bafe5d99965c48cc9d5c7cf024eeb495facc1e/dyno.go#L563-L601
+// License: Apache 2.0 - Copyright 2017 Andras Belicza
+// ConvertMapI2MapS walks the given dynamic object recursively, and
+// converts maps with interface{} key type to maps with string key type.
+// This function comes handy if you want to marshal a dynamic object into
+// JSON where maps with interface{} key type are not allowed.
+//
+// Recursion is implemented into values of the following types:
+//   -map[interface{}]interface{}
+//   -map[string]interface{}
+//   -[]interface{}
+//
+// When converting map[interface{}]interface{} to map[string]interface{},
+// fmt.Sprint() with default formatting is used to convert the key to a string key.
+func convertMapI2MapS(v interface{}) interface{} {
+	switch x := v.(type) {
+	case map[interface{}]interface{}:
+		m := map[string]interface{}{}
+		for k, v2 := range x {
+			switch k2 := k.(type) {
+			case string: // Fast check if it's already a string
+				m[k2] = convertMapI2MapS(v2)
+			default:
+				m[fmt.Sprint(k)] = convertMapI2MapS(v2)
+			}
+		}
+		v = m
+
+	case []interface{}:
+		for i, v2 := range x {
+			x[i] = convertMapI2MapS(v2)
+		}
+
+	case map[string]interface{}:
+		for k, v2 := range x {
+			x[k] = convertMapI2MapS(v2)
+		}
+	}
+
+	return v
+}
+
+func LoadYmlFileAsJson(path string) ([]byte, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+
+	m := make(map[interface{}]interface{})
+	err = yaml.Unmarshal(data, &m)
+	if err != nil {
+		return nil, err
+	}
+
+	j, err := json.Marshal(convertMapI2MapS(m))
+	if err != nil {
+		return nil, err
+	}
+
+	return j, nil
+}