Support multiple property names in `discriminator` keyword

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

8.6

**What problem do you want to solve?**

My schema depends on value of two fields, so I wanted to implement something like this:

```
{
type: 'object',
discriminator: { propertyName: ['a', 'b'] }
required: ['a', 'b'],
oneOf: [
{
properties: {
a: { const: true }
b: { const: 'b' }
c: { type: 'string' }
}
},
{
properties: {
a: { const: false }
b: { const: 'bb' }
c: { type: 'number' }
}
}
...
]
}
```

**What do you think is the correct solution to problem?**

Add ability to define multiple properties in `discriminator`

**Will you be able to implement it?**

I am not sure...