Allow to annotate class constants as properties

[DerManoMann]

Thanks to @cdaguerre for the idea and initial work; replaces #1147.

• adds support to annotate class constants, see example below
  related to Is annotation of class constants supported in v4? #1082)
• adds support for the const keyword to attributes
• handles converting a const to a single value enum for OAS 3.0.0

1. Attributes on class constants

use OpenApi\Attributes as OA;

#[OA\Schema()]
class Airport
{
    #[OA\Property(property='kind')]
    public const KIND = 'Airport';
}

2. Support the const keyword in attributes

use OpenApi\Attributes as OA;

#[OA\Schema()]
class Airport
{
    #[OA\Property(
        property: 'kind',
        type: 'string',
        const: 'Airport'
    )]
    public string $kind = 'Airport';
}

This ^^code is equivalent to the example above in terms of generated documentation.

3. Backport const type properties for versions < 3.1

The const keyword was introduced in OAS 3.1, eg.

components:
  schemas:
    Airport:
        properties:
          kind:
            type: string
            const: Airport

In OAS < 3.1, the recommended way of doing this was to declare a single-valued enum instead, eg.

components:
  schemas:
    Airport:
        properties:
          kind:
            type: string
            enum: 
              - Airport