Document cli configuration via JSON

[Nadegamra]

It is possible to configure some things via JSON file such as:

• api url
• issuer url
• client id
• client secret

However, this does not seem to be documented anywhere. Relevant code:

itwin-cli/src/extensions/configuration.ts

Lines 15 to 26 in 4f6f576

	constructor(configDir: string) {
	const configPath = path.join(configDir, "config.json");
	if (fs.existsSync(configPath)) {
	const file = fs.readFileSync(configPath, "utf8");
	const configJson = JSON.parse(file);
	this.apiUrl = configJson.apiUrl;
	this.clientId = configJson.clientId;
	this.clientSecret = configJson.clientSecret;
	this.issuerUrl = configJson.issuerUrl;
	}

Location of this file in different operating systems should also be documented. Necessary info can be found here: https://oclif.io/docs/config/