We'd like to let users change their plan directly from console or the CLI, which means we need to teach `PlansStorage#set` how to update Stripe.

I've introduced a new `BillingProvider` interface to start abstracting Stripe functionality and especially to make it easier to test systems that interact with Stripe. I have not done a thorough audit of the various ways we interact with Stripe, but I'd like to move toward having all Stripe interactions go through `BillingProvider`.

The linter is currently failing because I made the `account` field in the the billing customer schema optional and a fair amount of the billing logic relies on it being not-`undefined`. I could just go in and update logic to deal with undefined `account` but wanted to get some feedback from @alanshaw before I dig into that: I can see arguments for either option (roughly "we might want to track customers who don't have an account set up in Stripe" vs "the point of this table is to track Stripe accounts") but testing will require a bit more work if we want to preserve non-optionality (since the test stores don't get initialized from the test billing provider in the same way - in production they get initialized via the webhook handler, which doesn't currently have an analog in testing code). Either of the options here will require a bit more work, so I'd love to know if anyone has strong feelings before I finish this off.

linter still failing because I'm returning unexpected error types, need to update w3up one more time..

it gets initialized before our system, so should always have a record.

Co-authored-by: Alan Shaw <alan.shaw@protocol.ai>

also respect no-implicit-coercion

found this in the organization secrets in GitHub, so let's re-use it

per https://docs.github.com/en/actions/learn-github-actions/variables#using-the-vars-context-to-access-configuration-variable-values

This reverts commit d8ad82d.

I also rolled this in Stripe

I was not doing it correctly, which is I think why the integration tests are failing

I updated them in the Stripe test environment to match what we use in production, since we don't support using different plan names across the board.