Support callable signing_secrets for dynamic configuration

[kristinemcbride]

Why?
Multi-tenant Rails applications often need to use different Stripe webhook signing secrets for different tenants/regions. Currently, signing_secrets must be set to a static array at boot time, which doesn't work well for applications where the correct signing secrets are only known at request time.
This change allows signing_secrets to accept a callable (lambda/proc) that is evaluated on each webhook request, enabling dynamic resolution of signing secrets based on the current request context.

What?

• Added a custom getter for signing_secrets that resolves callable values (lambdas/procs) before returning
• Added documentation explaining the callable support
• Added tests for callable signing_secrets configuration
  After this change, applications can configure StripeEvent like this:

StripeEvent.signing_secrets = -> { CurrentTenant.stripe_signing_secrets }

See Also

activerecord-tenanted - This is the gem I am using that enables multi-tenant Rails applications, which is an example use case for this feature.