Re-add support for configuring server.host: "0"

Support for this was lost in elastic#85406.

Fixes elastic#86716

src/core/server/http/http_config.ts

@@ -195,7 +195,13 @@ export class HttpConfig {
     rawExternalUrlConfig: ExternalUrlConfig
   ) {
     this.autoListen = rawHttpConfig.autoListen;
-    this.host = rawHttpConfig.host;
+    // TODO: Consider dropping support for '0' in v8.0.0. This value is passed
+    // to hapi, which validates it. Prior to hapi v20, '0' was considered a
+    // valid host, however the validation logic internally in hapi was
+    // re-written for v20 and hapi no longer considers '0' a valid host. For
+    // details, see:
+    // https://github.com/elastic/kibana/issues/86716#issuecomment-749623781
+    this.host = rawHttpConfig.host === '0' ? '0.0.0.0' : rawHttpConfig.host;
     this.port = rawHttpConfig.port;
     this.cors = rawHttpConfig.cors;
     this.customResponseHeaders = Object.entries(rawHttpConfig.customResponseHeaders ?? {}).reduce(