Support for custom CA path in _get_custom_ca_certificate_location function

Hi,

It would be helpful if the CA cert path could be set to a custom location like `/etc/ssl/certs/ca-certificates.crt` or `/etc/pki/tls/certs/ca-bundle.crt` which are common paths for the systems CA cert bundles. Currently the [_get_custom_ca_certificate_location](https://github.com/bitwarden/splunk/blob/main/src/bitwarden_api.py#L41) function only supports placing CA certs in `$SPLUNK_HOME/etc/auth/bitwarden_event_logs_cacerts.pem`. I was also unable to find any documentation regarding the placement of CA certs in the event TLS validation is required. If this is something that can be changed, I can submit a PR. Thank you!

Possible solution using an environment variable called BITWARDEN_APP_CACERTS

```python3
def _get_custom_ca_certificate_location() -> Optional[str]:
path = os.environ.get(
"BITWARDEN_APP_CACERTS",
os.path.join(
os.environ.get("SPLUNK_HOME", ""),
"etc", "auth", "bitwarden_event_logs_cacerts.pem"
)
)
return path if os.path.isfile(path) else None
```

Read src/bitwarden_api.py, starting with _get_custom_ca_certificate_location, then trace where its returned path is used for event TLS validation. Done means a configured custom CA path such as BITWARDEN_APP_CACERTS is supported when the file exists, with the current default behavior preserved when it is not set.

Written by the indexing model from the issue text.