FR: Make enode persistent among restarts
Nimbus-eth1 currently initiates a new enode each time the service is restarted. This is inconsistent behaviour, as all other ELs will keep the enode as long as their db is not wiped, and init a new enode if the db is wiped.

It would be good to write nodekey to disk - probably where the db lives, which could be used as a seed to init the enode.

One way to set this now is using `--net-key=` but unfortunately this seems to have some bugs among restarts.

```sh
INF 2024-10-22 07:28:42.020+00:00 Starting metrics HTTP server               address=0.0.0.0 port=9001
INF 2024-10-22 07:28:42.025+00:00 Writing genesis to DB
INF 2024-10-22 07:28:42.025+00:00 Database initialized                       base="(0xba12b41c973893687480edec78579303aa951e2fd73ae7bb753753ff6d74652b, 0)" finalized="(0xba12b41c973893687480edec78579303aa951e2fd73ae7bb753753ff6d74652b, 0)" head="(0xba12b41c973893687480edec78579303aa951e2fd73ae7bb753753ff6d74652b, 0)"
/root/nimbus-eth1/nimbus/sync/protocol/snap/snap_types.nim(844) main
/root/nimbus-eth1/nimbus/sync/protocol/snap/snap_types.nim(832) NimMain
/root/nimbus-eth1/nimbus/nimbus_execution_client.nim(219) _ZN23nimbus_execution_client3runE3refIN11nimbus_desc26NimbusNodecolonObjectType_EEN6config10NimbusConfE.constprop.0
/root/nimbus-eth1/nimbus/nimbus_execution_client.nim(53) _ZN23nimbus_execution_client14manageAccountsE3refIN11nimbus_desc26NimbusNodecolonObjectType_EEN6config10NimbusConfE
/root/nimbus-eth1/vendor/nim-serialization/serialization.nim(90) _ZN7manager13loadKeystoresE3varIN7manager15AccountsManagerEE6string
/root/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/pure/json.nim(516) _ZN4json7X5BX5D_E3refIN4json11JsonNodeObjEE6string
/root/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/std/assertions.nim(41) _ZN10assertions16failedAssertImplE6string
/root/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/std/assertions.nim(36) _ZN10assertions11raiseAssertE6string
/root/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/system/fatal.nim(53) _ZN10assertions8sysFatalE8typeDescIN10exceptions15AssertionDefectEE6string
/root/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/system/excpt.nim(345) _ZN6system18rawWriteStackTraceE3varI3seqIN6system15StackTraceEntryEEE
/root/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/system/stacktraces.nim(62) _ZN11stacktraces30auxWriteStackTraceWithOverrideE3varI3seqIN6system15StackTraceEntryEEE
Error: unhandled exception: /root/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/pure/json.nim(515, 9) `node.kind == JObject`  [AssertionDefect]
```