Add optional appInfo to StripeClient config

[helenye-stripe]

This adds $appInfo as a valid key in $config when initializing a StripeClient. $appInfo expects the type

array{name: string, version?: string, url?: string, appPartnerId?: string}

It passes this in to APIRequestor as an init parameter with default null, and will be set in headers if not null.

Tests verify that we fall back to global configuration if not passed into StripeClient, by verifying the User-Agent header.

Changelog

• StripeClient can now accept $appInfo as a $config option, so AppInfo can be set per-client. If not passed in, will fall back on the global AppInfo set by Stripe::setAppInfo().
  • The config expects $appInfo to be of type array{name: string, version?: string, url?: string, partner_id?: string}

[anniel-stripe]

LGTM! Thanks for adding support for this!