Allow saved credit cards with a source ID (`src_`) to be used/displayed with the new checkout experience

[mattallan]

Fixes #3498

Changes proposed in this Pull Request:

As part of updating/migrating SEPA tokens over to pm_ IDs, we also worked on a change (see https://github.com/woocommerce/woocommerce-gateway-stripe/pull/3241) that removed support for displaying and storing any legacy src_ tokens. This fix was introduced because payment method requests with SEPA src_ tokens would failed.

From Stripe's docs on sources (https://docs.stripe.com/sources), they do however still support source tokens for card payments and so we should continue to support them in our extension.

While investigating https://github.com/woocommerce/woocommerce-gateway-stripe/issues/3498, we found that Google Pay and Apple Pay were still create valid credit card payment methods beginning with src_ and so our changes to remove/hide all saved cards with sources unexpectedly impacted customers that purchased using our Payment Request buttons.

The changes in this PR brings back support for displaying and storing payment methods starting with src_ but only for credit cards.

Proposed changes:

• df909d63f8cab522a6c486c53a997b16dbe50548 - Introduces a new helper function to validate payment method IDs (i.e. only allow source IDs if the payment method is 'card')
• d6bc810179e52a1bda457ef19a4b46018dc285be - Don't delete any saved sources when the type is 'CC'
• 3f03fd17b497b879d9936935f7a26da8f4641960 - Allow for legacy sources to be stored in the woocommerce_payment_tokens table if the payment method type is 'card'

[!NOTE] In @james-allan 's PR https://github.com/woocommerce/woocommerce-gateway-stripe/pull/3510, he's updating our Payment Request buttons implementation so that any new saved cards will be created as pm_ IDs.

In this PR, I'm fixing the issue for stores with existing customers with these saved legacy tokens.

Testing instructions

For the purpose of testing, I removed all my existing saved cards via the My Account > Payment methods page.

1. Enable WooCommerce Subscriptions and create a simple subscription product
2. Enable New Checkout experience and Apple Pay/Google Pay payment method
3. Make sure you're not using the new ECE feature (_wcstripe_feature_ece is set to 'no' in wp-options)
4. While on trunk, go to My Account > payment methods and confirm you have no saved payment methods
5. Purchase a subscription product using Google Pay or Apple Pay
6. From the stripe dashboard, navigate to the latest transaction and notice the payment method is a src token:
7. Navigate to My Account > Payment methods and notice there's no saved card.
8. Look in wp_woocommerce_payment_tokens table and notice there's no saved stripe tokens.
9. Checkout this branch
10. Refresh the My Account > Payment methods page to pull the latest saved methods from Stripe.
11. Confirm you now have a saved credit card.
12. Check the wp_woocommerce_payment_tokens table again and confirm you now see the saved src_ token with type 'CC'.

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• [x] Covered with tests (or have a good reason not to test in description ☝️)
• [x] Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• [ ] Tested on mobile (or does not apply)

Post merge

• [ ] Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• [ ] Added the needs docs label (or does not apply)
• [ ] Included this PR in the Release Thread scope (or does not apply)