Add a basic web server

[imobachgs]

Trello: https://trello.com/c/msRHsnpV/3570-3-build-a-basic-agama-web-server

This PR introduces the web server for Agama's 2024 architecture. At this point, it implements:

• A /ping endpoint.
• A simple WebSocket to receive progress events. It is just a proof-of-concept and even the format should be adapted.
• Tracing/logging.

Additionally, it is able to export the description of the API using OpenAPI.

Usage

The new binary implements two subcommands: serve and openapi.

Running the server

$ agama-web-server serve --help
Start the API server

Usage: agama-web-server serve [OPTIONS]

Options:
      --address <ADDRESS>  Address to listen on (default: "0.0.0.0:3000") [default: 0.0.0.0:3000]
  -h, --help               Print help

Generating the OpenAPI documentation

$ agama-web-sever openapi
{
  "openapi": "3.0.3",
  "info": {
    "title": "agama-dbus-server",
    "description": "Agama web API description",
    "license": {
      "name": ""
    },
    "version": "0.1.0"
  },
...

Additionally, it adds a new agama-web-server package to the RPM spec file.

To do

• Improve logging/tracing.
• Expose the API documentation (openAPI?).

Out of scope

• Better error handling.
• Read the D-Bus address from /run/agama/bus. We should do pretty much the same for agama-cli.
• Improve events format.

Testing

• Added a new Rus integration test
• Tested manually

[coveralls]

coverage: 73.938% (-0.09%) from 74.031%
when pulling 6020c5d on http-server
into be9bb78 on master.

[teclator]

LGTM