Allow providing an Ajv instance to Validator

[Sebastian Lenartowicz]

AJV has a plugin system, along the lines of the decorator pattern - each plugin exposes a function that modifies an instantiated Ajv object. However, because the Validator class only accepts AjvOptions and then constructs its own, applying plugins requires modifying it at runtime. This is fine (and expected, according to the tests) in straight JavaScript, but in TypeScript violates the provided types, because the Ajv is not included in the class definition:

// Plugin
import formats from "ajv-formats-draft2019";

const ajv = formats(new Ajv({allErrors: true}));
const validator = new Validator({});
validator.ajv = ajv;

// Yields: 
// TS2339: Property 'ajv' does not exist on type 'Validator'.

Either way, this seems a brittle approach, prone to breaking as the library changes. A backwards-compatible solution to this might be something like:

export class Validator {
    // Allow passing an Ajv to the constructor
    constructor(options: AjvOptions | Ajv);

    // Expose the ajv to match the behaviour of the JS implementation
    ajv: Ajv;
}

If a non-Ajv object is provided, the existing behaviour is kept, but otherwise the validator can just use the provided Ajv wholesale.

[Sebastian Lenartowicz]

Right, I realized that this one's basically a duplicate of #67.