`/_matrix/client/v3/register` fails over unix socket

This issue has been migrated from [#16742](https://github.com/matrix-org/synapse/issues/16742).

---

### Description

cp. #16404.

I had a quick glance at the code; Since the IP may be necessary later during execution I couldn't come up with a PR that just replaces the occurences with `.get_client_ip_if_available()` in `register.py`

### Steps to reproduce

cp. #16404

### Homeserver

selfhosted

### Synapse Version

1.95.1

### Installation Method

Other (please mention below)

### Database

postgresql

### Workers

Single process

### Platform

archlinux arm

### Configuration

_No response_

### Relevant log output

```shell
Traceback (most recent call last):
File "/usr/lib/python3.11/site-packages/synapse/http/server.py", line
326, in _async_render_wrapper
callback_return = await self._async_render(request)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/synapse/http/server.py", line
538, in _async_render
callback_return = await raw_callback_return
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/synapse/rest/client/_base.py"
, line 102, in wrapped
return await orig(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/synapse/rest/client/register.
py", line 436, in on_POST
client_addr = request.getClientAddress().host
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'UNIXAddress' object has no attribute 'host'
```

### Anything else that would be useful to know?

_No response_