osbuild2: add org.osbuild.inline source

Add support for the `org.osbuild.inline` source via a new `InlineSource`
and the corresponding `InlineSourceItem`. A utility function `AddItem`
can be used to conveniently add items based on data (as strings).

internal/osbuild2/inline_source.go

@@ -0,0 +1,41 @@
+package osbuild2
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+)
+
+type InlineSource struct {
+	Items map[string]InlineSourceItem `json:"items"`
+}
+
+func (InlineSource) isSource() {}
+
+type InlineSourceItem struct {
+	Encoding string `json:"encoding"`
+	Data     string `json:"data"`
+}
+
+func NewInlineSource() *InlineSource {
+	return &InlineSource{
+		Items: make(map[string]InlineSourceItem),
+	}
+}
+
+// AddItem a new item to the source. Well hash and encode that data
+// and return the checksum.
+func (s *InlineSource) AddItem(data string) string {
+
+	dataBytes := []byte(data)
+
+	encoded := base64.StdEncoding.EncodeToString(dataBytes)
+	name := fmt.Sprintf("sha256:%x", sha256.Sum256(dataBytes))
+
+	s.Items[name] = InlineSourceItem{
+		Encoding: "base64",
+		Data:     encoded,
+	}
+
+	return name
+}