Add Journal EventLog to Manager

In order to get access to the EventLog on multi-host platforms,
add Journal EventLog to Manager.
This implementation is based on the discussion we had on
patch 76319

Add Journal EventLog to Manager

In order to get access to the EventLog on multi-host platforms,
add Journal EventLog to Manager.
This implementation is based on the discussion we had on
patch 76319 [1].

TLDR: On multi-host, we technically would have to split the event log
on a per host node basis, so that each host node has its own
specific event log.

However, this is currently not supported so we had to decide,
whether we put it on a specific ComputerSystem, or refactor the current
implementation of the EventLog, to allow for the EventLog LogService to
be part of the Managers resource.
We chose the latter one, because a), it is not clear on which
ComputerSystem to put the EventLog, as long as we aren't splitting the
event log per host node, and b), if that particular
ComputerSystem is not existing at runtime, there would be no access to
the EventLog at all.

This feature can be enabled with the redfish-eventlog-location meson
option. By default it is set to 'systems', which translates to the
EventLog being under the Systems resource.
To enable the EventLog under the Managers resource set

```
-Dredfish-eventlog-location=managers
```
This in turn, disables the EventLog under the ComputerSystem resource.

Tested: Redfish validation succeeded for both ComputerSystem and
Managers tree.

```
curl command:
curl -w "@curl-format.txt" -c cjar -b cjar -k -X GET 'https://'"${BMC}"':4443/redfish/v1/'"$ROUTE"'' \
-H 'X-Auth-Token: '"$BMCWEB_SESSION_TOKEN"''

GET /redfish/v1/Managers/bmc/LogServices
{
"@odata.id": "/redfish/v1/Managers/bmc/LogServices",
"@odata.type": "#LogServiceCollection.LogServiceCollection",
"Description": "Collection of LogServices for this Manager",
"Members": [
{
"@odata.id": "/redfish/v1/Managers/bmc/LogServices/Journal"
},
{
"@odata.id": "/redfish/v1/Managers/bmc/LogServices/EventLog"
}
],
"Members@odata.count": 2,
"Name": "Open BMC Log Services Collection"
}

GET /redfish/v1/Managers/bmc/LogServices/EventLog
{
"@odata.id": "/redfish/v1/Managers/bmc/LogServices/EventLog",
"@odata.type": "#LogService.v1_2_0.LogService",
"Actions": {
"#LogService.ClearLog": {
"target": "/redfish/v1/Managers/bmc/LogServices/EventLog/Actions/LogService.ClearLog"
}
},
"DateTime": "2025-09-24T15:22:36+00:00",
"DateTimeLocalOffset": "+00:00",
"Description": "Manager Event Log Service",
"Entries": {
"@odata.id": "/redfish/v1/Managers/bmc/LogServices/EventLog/Entries"
},
"Id": "EventLog",
"Name": "Event Log Service",
"OverWritePolicy": "WrapsWhenFull"
}

GET /redfish/v1/Managers/bmc/LogServices/EventLog/Entries
{
"@odata.id": "/redfish/v1/Managers/bmc/LogServices/EventLog/Entries",
"@odata.type": "#LogEntryCollection.LogEntryCollection",
"Description": "Collection of Manager Event Log Entries",
"Members": [
{
"@odata.id": "/redfish/v1/Managers/bmc/LogServices/EventLog/Entries/1730009576",
"@odata.type": "#LogEntry.v1_9_0.LogEntry",
"Created": "2024-10-27T06:12:56+00:00",
"EntryType": "Event",
"Id": "1730009576",
"Message": "Host system DC power is off",
"MessageArgs": [],
"MessageId": "OpenBMC.0.1.DCPowerOff",
"Name": "Manager Event Log Entry",
"Severity": "OK"
},
...
],
"Members@odata.count": 2820,
"Members@odata.nextLink": "/redfish/v1/Managers/bmc/LogServices/EventLog/Entries?$skip=1000",
"Name": "Manager Event Log Entries"
}

GET /redfish/v1/Managers/bmc/LogServices/EventLog/Entries/1730009576
{
"@odata.id": "/redfish/v1/Managers/bmc/LogServices/EventLog/Entries/1730009576",
"@odata.type": "#LogEntry.v1_9_0.LogEntry",
"Created": "2024-10-27T06:12:56+00:00",
"EntryType": "Event",
"Id": "1730009576",
"Message": "Host system DC power is off",
"MessageArgs": [],
"MessageId": "OpenBMC.0.1.DCPowerOff",
"Name": "Manager Event Log Entry",
"Severity": "OK"
}
```
ClearLog action:
Log files are being successfully deleted from /var/log

[1] https://gerrit.openbmc.org/c/openbmc/bmcweb/+/76319

Change-Id: If5b4fe10151b6bfd28a1b49c41f8cfcec1b9132c
Signed-off-by: Oliver Brewka <oliver.brewka@9elements.com>

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