[HTTP Server] support TLS config hot reload via SIGHUP (#171823)

## Summary

Fix #54368

Add support for hot reloading the Kibana server's TLS configuration,
using the same `SIGHUP`-based reload signal, as already implemented for
other parts of the Kibana configuration (e.g `logging`)

**Note:**
- hot reloading is only supported for the server TLS configuration
(`server.ssl`), not for the whole `server.*` config prefix
- swaping the certificate files (without modifying the kibana config
itself) is supported
- it is not possible to toggle TLS (enabling or disabling) without
restarting Kibana
- hot reloading requires to force the process to reload its
configuration by sending a `SIGHUP` signal

### Example / how to test

#### Before

```yaml
server.ssl.enabled: true
server.ssl.certificate: /path-to-kibana/packages/kbn-dev-utils/certs/kibana.crt
server.ssl.key: /path-to-kibana/packages/kbn-dev-utils/certs/kibana.key
```

<img width="550" alt="Screenshot 2023-11-23 at 15 11 28"
src="https://github.com/elastic/kibana/assets/1532934/1226d161-a9f2-4d62-a3de-37161829f187">

#### Changing the config

```yaml
server.ssl.enabled: true
server.ssl.certificate: /path-to-kibana/packages/kbn-dev-utils/certs/elasticsearch.crt
server.ssl.key: /path-to-kibana/packages/kbn-dev-utils/certs/elasticsearch.key
```

```bash
kill -SIGHUP {KIBANA_PID}
```

<img width="865" alt="Screenshot 2023-11-23 at 15 18 21"
src="https://github.com/elastic/kibana/assets/1532934/c9412b2e-d70e-4cf0-8eaf-4db70a45af60">

#### After

<img width="547" alt="Screenshot 2023-11-23 at 15 18 43"
src="https://github.com/elastic/kibana/assets/1532934/c839f04f-4adb-456d-a174-4f0ebd5c234c">

## Release notes

It is now possible to hot reload Kibana's TLS (`server.ssl`)
configuration by updating it and then sending a `SIGHUP` signal to the
Kibana process.

Note that TLS cannot be toggled (disabled/enabled) that way, and that
hot reload only works for the TLS configuration, not other properties of
the `server` config prefix.

---------

Co-authored-by: kibanamachine <[email protected]>

packages/core/http/core-http-server-internal/src/http_server.test.mocks.ts

@@ -0,0 +1,18 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+export const setTlsConfigMock = jest.fn();
+
+jest.doMock('@kbn/server-http-tools', () => {
+  const actual = jest.requireActual('@kbn/server-http-tools');
+  return {
+    ...actual,
+    setTlsConfig: setTlsConfigMock,
+    createServer: jest.fn(actual.createServer),
+  };
+});