AMQP message format is very inconvenient

[Skia]

Autopkgtest-cloud uses a RabbitMQ instance to manage the queue of tests that needs to be run. Those tests requests are coming from Britney (and security britney too), the developers through the `/request.cgi` endpoint, and the admins with the `run-autopkgtest` script.

Those tests requests need to convey some amount of information, like the package name, the triggers, the requester, PPAs, etc...

The current defacto format looks like the following [1]:
```
hello\n{"triggers": ["hello/1.2.3ubuntu2"], "submit-time": "2024-02-22 01:56:14"}
```
More data can be embedded in the JSON, but it's not the point of this bug.

The issue with this format is this: `package_name` + `\n` + `some JSON object`.
That means that:
* everything that pushes or read test requests to/from the queue need to speak JSON
* every time something needs to handle a test requests, it's not **only** JSON, and the prepended `package_name\n` needs to be dealt with

While the current format **might** have a **slight** advantage when building **some** particular shell pipelines, it's in overall just a custom format that is mostly JSON, and could easily be 100% JSON anyway.

The proposition here is to start an effort to convert the AMQP message format to 100% JSON by adding the package name in a `package` key to the existing JSON object, making the message presented above look like the following:
```
{"package": "hello", "triggers": ["hello/1.2.3ubuntu2"], "submit-time": "2024-02-22 01:56:14"}
```
Gain:
* simplify existing code by just parsing JSON instead of custom format
* simplify the development of tooling by only requiring JSON parsing, which is already implemented in most languages
* have a

For this transition to be as smooth as possible, the steps could be the following:
1. Make the autopkgtest-cloud worker code aware of the new format, by discriminating on the first character being a `{` or not.
2. Make `request.cgi` able to queue tests with the new message format, possibly with a manual flag at first, like `&amqp-format=new` in the URL, that could be non-default at first, then later turned to be the default, but still easily removable if some tests requests go wrong.
3. Make Britney use the new format after a while of stable usage by the developers.
4. One cycle or year later, when everything has completely stabilized, completely remove the old format support from all the scripts that deal with it.

This may sound like a big amount of work, but it's in fact only some tiny portions of code, quite simple, that are just a bit scattered across multiple files and repositories. Also, if performed in the right order, none of those changes should require any coordination with the rest of the pieces, which also means that both rollbacks or iteration are quite easy.

[1]: it's "defined" here: https://autopkgtest-cloud.readthedocs.io/en/latest/architecture.html#test-request-format
     One problem being that the `\n` sometimes is a single space, and nothing is really enforced anywhere here, making it complicated to build tools that iterate over the whole queue, because there's no consistency.