Adds the `org.matrix.msc3575.proxy` field (used for configuring sliding
sync) to /.well-known/matrix/client when Dendrite is serving that
endpoint and `well_known_sliding_sync_proxy` has been configured.
ie. Config values of:
``` yaml
global:
well_known_client_name: https://example.com
well_known_sliding_sync_proxy: https://syncv3.example.com
```
results in a /.well-known/matrix/client of:
``` json
{
"m.homeserver": {
"base_url": "https://example.com"
},
"org.matrix.msc3575.proxy": {
"url": "https://syncv3.example.com"
}
}
```
If `well_known_sliding_sync_proxy` is not provided, the json provided by
/.well-known/matrix/client does not include the proxy field.
ie.
``` json
{
"m.homeserver": {
"base_url": "https://example.com"
}
}
```
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