walproposer: basic infra to enable generations (#11002)

## Problem

Preparation for https://github.com/neondatabase/neon/issues/10851

## Summary of changes

Add walproposer `safekeepers_generations` field which can be set by
prefixing `neon.safekeepers` GUC with `g#n:`. Non zero value (n) forces
walproposer to use generations. In particular, this also disables
implicit timeline creation as timeline will be created by storcon. Add
test checking this. Also add missing infra: `--safekeepers-generation`
flag to neon_local endpoint start + fix `--start-timeout` flag: it
existed but value wasn't used.

compute_tools/src/config.rs

@@ -1,5 +1,7 @@
+use std::fmt::Write as FmtWrite;
 use std::fs::{File, OpenOptions};
 use std::io;
+use std::io::Write;
 use std::io::prelude::*;
 use std::path::Path;
 
@@ -55,10 +57,20 @@ pub fn write_postgres_conf(
         writeln!(file, "neon.stripe_size={stripe_size}")?;
     }
     if !spec.safekeeper_connstrings.is_empty() {
+        let mut neon_safekeepers_value = String::new();
+        tracing::info!(
+            "safekeepers_connstrings is not zero, gen: {:?}",
+            spec.safekeepers_generation
+        );
+        // If generation is given, prepend sk list with g#number:
+        if let Some(generation) = spec.safekeepers_generation {
+            write!(neon_safekeepers_value, "g#{}:", generation)?;
+        }
+        neon_safekeepers_value.push_str(&spec.safekeeper_connstrings.join(","));
         writeln!(
             file,
             "neon.safekeepers={}",
-            escape_conf_value(&spec.safekeeper_connstrings.join(","))
+            escape_conf_value(&neon_safekeepers_value)
         )?;
     }
     if let Some(s) = &spec.tenant_id {