[Breaking] Rename "payment method" to "payment option"

[yuki-stripe]

Summary & Motivation

This change precedes adding PaymentMethod. Unfortunately, we already have a type that collides with this name, so I'm renaming it all to PaymentOption.

I've opted to aggressively rename most all references to 'payment method' - this means possibly more work for users to upgrade, but will make the upcoming usage of PaymentMethod clear and consistent.

Testing

All StripeIOS Tests pass, all projects build.

[yuki-stripe]

Should I update MIGRATING.md in this commit as well?

[bg-stripe]

Looks good overall! A couple requests:

1. Let's update STPSDKVersion in STPAPIClient.h to 15.0.0 (major version update, since this is a breaking change). We should also update the version in the Info.plist.

2. Looks like there are references to an example backend tag called v14.0.0. That reminds me that we'll probably need to cut a new example backend version, v15.0.0 as part of this release (currently the backend creates a charge with a source). cc @mshafrir-stripe

3. Let's add a new changelog entry in CHANGELOG.md for 15.0.0 (maybe leaving a todo for the release date), and describe the breaking changes here. It's much easier to add changelog entries for breaking changes as part of incremental PRs, rather than going back later.

[bg-stripe]

Should I update MIGRATING.md in this commit as well?

@yuki-stripe ah yeah good call, let's do that too.

[bg-stripe]

lgtm!

Fixed all issues