[Enhancement Request] Allow configuring a different install directory for Endpoint

Hi there --

I was just trying to enable the Elastic Defend integration in my Elastic deployment, and found that currently, it appears to require being installed into `/opt/Elastic`:

```
# elastic-agent status
┌─ fleet
│ └─ status: (HEALTHY) Connected
└─ elastic-agent
├─ status: (DEGRADED) 1 or more components/units in a failed state
└─ endpoint-default
├─ status: (FAILED) failed install endpoint service: 2024-03-19 17:56:48: error: Internal.cpp:311 Unable to create directory /opt/Elastic: exit status 214
├─ endpoint-default
│ └─ status: (FAILED) failed install endpoint service: 2024-03-19 17:56:48: error: Internal.cpp:311 Unable to create directory /opt/Elastic: exit status 214
└─ endpoint-default-36297671-98bd-4b89-92b3-61d772686db4
└─ status: (FAILED) failed install endpoint service: 2024-03-19 17:56:48: error: Internal.cpp:311 Unable to create directory /opt/Elastic: exit status 214
```

This is an issue for us in our environment as we have many hosts that are stateless, often without a harddrive in them, where `/opt` is almost always a RO mount from another host.

Ideally, we would be able to configure a different (local) path on those stateless nodes. It's very likely that this path might be a RAM filesystem which would disappear if the host rebooted, so ideally there would not be very much state in that directory, and it would be able to easily regenerate itself by re-enrolling / re-starting the elastic agent process.

I'd be happy to collaborate on how this might look and / or to do any early testing if that would be helpful. Cheers!