Gravity Forms Stripe Addon - Add filter to be able to set the plan_id before sending to Stripe

class-gf-stripe.php

public function get_subscription_plan_id( $feed, $payment_amount, $trial_period_days, $currency = '' ) {

		$safe_feed_name      = preg_replace( '/[^a-z0-9_\-]/', '', strtolower( $feed['meta']['feedName'] ) );
		$safe_billing_cycle  = $feed['meta']['billingCycle_length'] . $feed['meta']['billingCycle_unit'];
		$safe_trial_period   = $trial_period_days ? 'trial' . $trial_period_days . 'days' : '';
		$safe_payment_amount = $this->get_amount_export( $payment_amount, $currency );

		/*
		 * Only include the currency code in the plan id when the entry currency does not match the plugin currency.
		 * Ensures the majority of plans created before this change will continue to be used.
		 * https://stripe.com/docs/subscriptions/plans#working-with-local-currencies
		*/
		if ( ! empty( $currency ) && $currency === GFCommon::get_currency() ) {
			$currency = '';
		}

		$plan_id = implode( '_', array_filter( array( $safe_feed_name, $feed['id'], $safe_billing_cycle, $safe_trial_period, $safe_payment_amount, $currency ) ) );

		$plan_id = apply_filters( 'gf_stripe_plan_id', $plan_id );

		$this->log_debug( __METHOD__ . '(): ' . $plan_id );

		return $plan_id;

}