[JsonStreamer] Add synthetic properties support

[mtarld]

Q	A
Branch?	7.4
Bug fix?	no
New feature?	yes
Deprecations?	no
Issues
License	MIT

This PR adds the support for virtual synthetic properties, which are basically values that are computed thanks to property metadata loaders only.
This could be useful for projects that are generating highly dynamic object shape, such as API Platform.

/cc @soyuka

[soyuka]

Really great improvement, this is quite painful for now in API Platform as we use id everywhere and sometimes the property doesn't exist.

[chalasr]

From https://www.php.net/manual/en/language.oop5.property-hooks.php

A property may be "backed" or "virtual". [...] A virtual property is one that has hooks and those hooks do not interact with the property itself. In this case, the hooks are effectively the same as methods, and the object does not use any space to store a value for that property.

Should we find another name?

[stof]

The reason for the name is actually similar to the reason in property hooks (a JsonStreamer property that is either backed by a PHP property or not). But it might indeed cause confusion, especially when the 2 different layers are closely tied to each other.
I don't have an idea for a good alternate name though.

[GromNaN]

"synthetic" means that it is created from nothing, or from transforming other things.

[chalasr]

I like it.

[mtarld]

Updated with the synthetic name instead. It's a nice name, I like it as well, thanks for the review.

[GromNaN]

Thank you @mtarld.