fix(stitch): do not remove unmerged fields from both isolated and non-isolated subschemas (#6105)

* fix(stitch): do not remove unmerged fields from both isolated and non-isolated subschemas

* Changesets

* Cleanup changeset

* v15 compat

* Skip tests for v15

* Hmm

* Yes

Author: ardatan

.changeset/big-horses-study.md

@@ -0,0 +1,6 @@
+---
+"@graphql-tools/stitch": patch
+"@graphql-tools/utils": patch
+---
+
+Handle fields in unmerged types as both isolated and non-isolated fields

.changeset/curvy-garlics-grin.md

@@ -0,0 +1,5 @@
+---
+"@graphql-tools/stitch": patch
+---
+
+Respect `specifiedByURL` in stitched schemas

.changeset/honest-beds-try.md

@@ -0,0 +1,5 @@
+---
+"@graphql-tools/utils": minor
+---
+
+Add `enumValueFilter` and `directiveFilter` to `filterSchema`