[FEATURE]: Add partner flow for PayPal

[ThisIsMani]

Feature Description

PayPal offers an onboarding flow, where merchants can be onboarded directly from a partner account. But for that, we need to change the authentication logic for PayPal request.

We should be able to take a 3rd field in the connector_account_details. So we need to support SignatureKey as well.

Possible Implementation

Add support for SignatureKey

Have you spent some time checking if this feature request has been raised before?

• I checked and didn't find a similar issue

Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

Are you willing to submit a PR?

Yes, I am willing to submit a PR!