Deffine NEON_SMGR to make it possible for extensions to use Neon SMG API (#4840)

## Problem

See https://neondb.slack.com/archives/C036U0GRMRB/p1689148023067319

## Summary of changes

Define NEON_SMGR in smgr.h

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

vendor/revisions.json

@@ -1,4 +1,4 @@
 {
-    "postgres-v15": "1220c8a63f00101829f9222a5821fc084b4384c7",
-    "postgres-v14": "ebedb34d01c8ac9c31e8ea4628b9854103a1dc8f"
+    "postgres-v15": "770c6dffc5ef6aac05bf049693877fb377eea6fc",
+    "postgres-v14": "da3885c34db312afd555802be2ce985fafd1d8ad"
 }