Remove Get/SetZenithCurrentClusterSize from Postgres core

[knizhnik]

Problem

See https://neondb.slack.com/archives/C04DGM6SMTM/p1711003752072899

Summary of changes

Move keeping of cluster size to neon extension

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

[github-actions]

2736 tests run: 2604 passed, 0 failed, 132 skipped (full report)

---

Flaky tests (5)

Postgres 16

• test_sharding_split_smoke: release
• test_create_churn_during_restart: debug

Postgres 15

• test_sharding_split_smoke: debug

Postgres 14

• test_sharding_split_smoke: release, debug

Code coverage* (full report)

• functions: 28.0% (6209 of 22164 functions)
• lines: 46.7% (43709 of 93657 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
bc3e1d5 at 2024-03-22T10:51:12.557Z :recycle:}

[petuhovskiy]

Build is failing due to Rust codegen from C structures, you need to add currentClusterSize initialization when walproposer::bindings::WalproposerShmemState is created in walproposer_api.rs

Compiling attachment_service v0.1.0 (/__w/neon/neon/control_plane/attachment_service)
error[E0063]: missing field `currentClusterSize` in initializer of `WalproposerShmemState`
   --> safekeeper/tests/walproposer_sim/walproposer_api.rs:243:36
    |
243 |             shmem: UnsafeCell::new(walproposer::bindings::WalproposerShmemState {
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `currentClusterSize`

For more information about this error, try `rustc --explain E0063`.
error: could not compile `safekeeper` (test "simple_test") due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `safekeeper` (test "random_test") due to 1 previous error
error: could not compile `safekeeper` (test "misc_test") due to 1 previous error

For example, I recently did a similar thing here https://github.com/neondatabase/neon/pull/7100/files#diff-c11d763f45c03fa45b4d3b82634b8ef48177e6f1ebe1098471078aea6b66753cR248

[knizhnik]

Build is failing due to Rust codegen from C structures, you need to add currentClusterSize initialization when walproposer::bindings::WalproposerShmemState is created in walproposer_api.rs

Compiling attachment_service v0.1.0 (/__w/neon/neon/control_plane/attachment_service)
error[E0063]: missing field `currentClusterSize` in initializer of `WalproposerShmemState`
   --> safekeeper/tests/walproposer_sim/walproposer_api.rs:243:36
    |
243 |             shmem: UnsafeCell::new(walproposer::bindings::WalproposerShmemState {
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `currentClusterSize`

For more information about this error, try `rustc --explain E0063`.
error: could not compile `safekeeper` (test "simple_test") due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `safekeeper` (test "random_test") due to 1 previous error
error: could not compile `safekeeper` (test "misc_test") due to 1 previous error

For example, I recently did a similar thing here https://github.com/neondatabase/neon/pull/7100/files#diff-c11d763f45c03fa45b4d3b82634b8ef48177e6f1ebe1098471078aea6b66753cR248

Thank you!
Fixed

[skyzh]

merging to unblock #7209