MM-13664 Cache external link metadata when populating post metadata (#10128)

* MM-13664 Added LinkMetadata types

* MM-13664 Use LinkMetadata when populating post metadata

* Fix unused import

* Fix index name on SQLite

* Finish adding unit tests

* Address feedback

* Increase max length of URL column to 2048 characters

model/post_metadata.go

@@ -3,6 +3,10 @@
 
 package model
 
+import (
+	"encoding/json"
+)
+
 type PostMetadata struct {
 	// Embeds holds information required to render content embedded in the post. This includes the OpenGraph metadata
 	// for links in the post.
@@ -28,3 +32,8 @@ type PostImage struct {
 	Width  int `json:"width"`
 	Height int `json:"height"`
 }
+
+func (o *PostImage) ToJson() string {
+	b, _ := json.Marshal(o)
+	return string(b)
+}