feat: specify env values from the parent to the nested state (#622)

* feat: specify env values from the parent to the nested state

Adds the `helmfiles[].environment.values` that accepts a mix of file pathes and inline dictes:

```yaml
helmfiles:
- path: path/to/nested/helmfile.yaml
  environment:
    values:
    - key1: val1
    - values.yaml
```

The values files are loaded in the context of the parent state file. For example, in case the above state file is located at `/path/to/helmfile.yaml`,
`values.yaml` is located at `/path/to/values.yaml` instead of `/path/to/nested/values.yaml`.

Resolves #523

* fix: multiple "bases" declarations yields duplicate releases

Fixes #615

* fix regression in double-rendering with env value overrides

The latest commit broke any state files like the below to NOT pass env value overrides at all:

```
helmfiles:
- path: nested/state.yaml
  environment:
    values:
    - overrides.yaml
```

This fixes the issue.

environment/environment.go

@@ -1,8 +1,45 @@
 package environment
 
+import (
+	"encoding/json"
+	"github.com/imdario/mergo"
+)
+
 type Environment struct {
 	Name   string
 	Values map[string]interface{}
 }
 
 var EmptyEnvironment Environment
+
+func (e Environment) DeepCopy() Environment {
+	bytes, err := json.Marshal(e.Values)
+	if err != nil {
+		panic(err)
+	}
+	var values map[string]interface{}
+	if err := json.Unmarshal(bytes, &values); err != nil {
+		panic(err)
+	}
+	return Environment{
+		Name:   e.Name,
+		Values: values,
+	}
+}
+
+func (e *Environment) Merge(other *Environment) (*Environment, error) {
+	if e == nil {
+		if other != nil {
+			copy := other.DeepCopy()
+			return &copy, nil
+		}
+		return nil, nil
+	}
+	copy := e.DeepCopy()
+	if other != nil {
+		if err := mergo.Merge(&copy, other, mergo.WithOverride); err != nil {
+			return nil, err
+		}
+	}
+	return &copy, nil
+}