Support for discriminator tag lookup in referenced schemas

**What version of Ajv you are you using?**
8.12.0

**What problem do you want to solve?**
If the discrimantor property needs to be evaluated in a object which contains a allOf instead of direct properties, the discriminator can not be evaluated. Also the mapping param should be used if one exists.

Example of such a mapping:
```
Song:
description: A song
type: object
oneOf:
- $ref: '#/components/schemas/InstrumentalSongObject'
- $ref: '#/components/schemas/PopSongObject'
discriminator:
propertyName: type
mapping:
INSTRUMENTAL: '#/components/schemas/InstrumentalSongObject'
POP: '#/components/schemas/PopSongObject'
BaseSong:
description: Base object
type: object
required:
- title
properties:
title:
description: The title
type: string
InstrumentalSong:
description: 'Instrumental Song'
required:
- type
type: object
properties:
type:
type: string
description: Song type
enum:
- INSTRUMENTAL
example: INSTRUMENTAL
band:
description: The band
type: string
PopSong:
description: 'Pop song'
required:
- type
type: object
properties:
type:
type: string
description: Song type
enum:
- POP
example: POP
artist:
description: The artist
type: string
InstrumentalSongObject:
description: A instrumental song
type: object
allOf:
- $ref: '#/components/schemas/BaseSong'
- $ref: '#/components/schemas/InstrumentalSong'
PopSongObject:
description: A pop song
type: object
allOf:
- $ref: '#/components/schemas/BaseSong'
- $ref: '#/components/schemas/PopSong'
```

**What do you think is the correct solution to problem?**
the getMapping function in the package discriminator needs to be extended tu support allOf for evaluation of the discriminator in the props of these subobjects.

**Will you be able to implement it?**
yes, PR will follow