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

store/sqlstore/store.go

@@ -96,4 +96,5 @@ type SqlStore interface {
 	Scheme() store.SchemeStore
 	TermsOfService() store.TermsOfServiceStore
 	UserTermsOfService() store.UserTermsOfServiceStore
+	LinkMetadata() store.LinkMetadataStore
 }