Add configuration option for sliding sync when hosting /.well-known/matrix/client (#3189)

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"
    }
}
```

1 files changed, 4 insertions, 0 deletions

diff --git a/setup/config/config_global.go b/setup/config/config_global.go
index 1622bf35..5b4ccf40 100644
--- a/setup/config/config_global.go
+++ b/setup/config/config_global.go
@@ -48,6 +48,10 @@ type Global struct {
 	// The server name to delegate client-server communications to, with optional port
 	WellKnownClientName string `yaml:"well_known_client_name"`
 
+	// The server name to delegate sliding sync communications to, with optional port.
+	// Requires `well_known_client_name` to also be configured.
+	WellKnownSlidingSyncProxy string `yaml:"well_known_sliding_sync_proxy"`
+
 	// Disables federation. Dendrite will not be able to make any outbound HTTP requests
 	// to other servers and the federation API will not be exposed.
 	DisableFederation bool `yaml:"disable_federation"`