Add safekeeper membership conf to control file. (#10196)

## Problem

https://github.com/neondatabase/neon/issues/9965

## Summary of changes

Add safekeeper membership configuration struct itself and storing it in
the control file. In passing also add creation timestamp to the control
file (there were cases where I wanted it in the past).

Remove obsolete unused PersistedPeerInfo struct from control file (still
keep it control_file_upgrade.rs to have it in old upgrade code).

Remove the binary representation of cfile in the roundtrip test.
Updating it is annoying, and we still test the actual roundtrip.

Also add configuration to timeline creation http request, currently used
only in one python test. In passing, slightly change LSNs meaning in the
request: normally start_lsn is passed (the same as ancestor_start_lsn in
similar pageserver call), but we allow specifying higher commit_lsn for
manual intervention if needed. Also when given LSN initialize
term_history with it.

test_runner/fixtures/utils.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import contextlib
+import dataclasses
 import json
 import os
 import re
@@ -21,6 +22,7 @@ import zstandard
 from psycopg2.extensions import cursor
 from typing_extensions import override
 
+from fixtures.common_types import Id, Lsn
 from fixtures.log_helper import log
 from fixtures.pageserver.common_types import (
     parse_delta_layer,
@@ -605,6 +607,22 @@ class PropagatingThread(threading.Thread):
         return self.ret
 
 
+class EnhancedJSONEncoder(json.JSONEncoder):
+    """
+    Default json.JSONEncoder works only on primitive builtins. Extend it to any
+    dataclass plus our custom types.
+    """
+
+    def default(self, o):
+        if dataclasses.is_dataclass(o) and not isinstance(o, type):
+            return dataclasses.asdict(o)
+        elif isinstance(o, Id):
+            return o.id.hex()
+        elif isinstance(o, Lsn):
+            return str(o)  # standard hex notation
+        return super().default(o)
+
+
 def human_bytes(amt: float) -> str:
     """
     Render a bytes amount into nice IEC bytes string.