Stripe Plugin does not always use the passed in "stripeSecretKey"

[cgilly2fast]

Describe the Bug

Title: Stripe Plugin Environment Variable Handling for Non-dotenv Configurations

Description
The current implementation of the Stripe plugin has a rigid dependency on process.env.STRIPE_SECRET_KEY, assuming it's set via dotenv. This causes issues when using alternative deployment configurations like SST, where environment variables are injected differently.

Current Behavior

1. The plugin currently looks for process.env.STRIPE_SECRET_KEY in three instances, ignoring the Stripe key that's passed into the plugin configuration.
2. This forces users to implement hacky workarounds, such as manually setting process.env.STRIPE_SECRET_KEY at the top of the payload config, which can lead to errors and is not a robust solution.

Current places Payload does not use the passed in Stripe secret key but only looks for the env var:

https://github.com/payloadcms/payload/blob/main/packages/plugin-stripe/src/hooks/createNewInStripe.ts

https://github.com/payloadcms/payload/blob/main/packages/plugin-stripe/src/hooks/deleteFromStripe.ts

https://github.com/payloadcms/payload/blob/main/packages/plugin-stripe/src/hooks/syncExistingWithStripe.ts

Ideal Behavior
The plugin should:

1. Prioritize the Stripe key passed through the plugin configuration
2. Fall back to or not use process.env.STRIPE_SECRET_KEY
3. Support different environment variable injection methods beyond dotenv

Proposed Solution
Refactor the plugin to implement a more flexible environment variable handling system:

// Example implementation
const getStripeKey = (config) => {
  if (config.stripeKey) {
    return config.stripeKey;
  }
  
  if (process.env.STRIPE_SECRET_KEY) {
    return process.env.STRIPE_SECRET_KEY;
  }
  
  throw new Error('No Stripe key provided. Please provide it either through plugin configuration or STRIPE_KEY environment variable.');
}

Use Case
When using SST (Serverless Stack) for deployment, environment variables are injected at runtime, and we cannot guarantee they'll be available through process.env.STRIPE_KEY. The current implementation makes it difficult to use the plugin in this and similar scenarios.

Impact
This issue affects anyone using the Stripe plugin with:

• SST
• Other serverless platforms with custom environment variable injection
• Custom environment variable management solutions
• Container-based deployments with different env variable patterns

Let me know if you need any additional information or clarification.

Link to the code that reproduces this issue

https://github.com/payloadcms/payload

Reproduction Steps

Example stripe plugin set up produces error:

 stripePlugin({
      stripeSecretKey: Resource.StripeSecretKey.value,
      sync: [
        {
          collection: 'billings',
          stripeResourceType: 'customers',
          stripeResourceTypeSingular: 'customer',
          fields: [
            {
              fieldPath: 'firm',
              stripeProperty: 'name',
            },
          ],
        },
      ],
    })

Which area(s) are affected? (Select all that apply)

plugin: stripe

Environment Info

**Additional Context**
- Using Payload CMS version: 3.17.1
- Using @payloadcms/plugin-stripe version: 3.17.1
- Deployment platform: SST