* feat: add `hasPaywall` getter to `Offering`
Returns `true` if the offering has either a v1 or a v2 paywall
* docs: update some methods references changing `PaywallData` with "paywall", as `PaywallData` only refers to v1 paywalls
* docs: improve reference of `Offering`'s `paywall` and `paywallComponents` to mention the new `hasPaywall` property
* lint
* tests: add unit tests that check the `hasPaywall` property of `Offering`
* api tests: add `paywallComponents` and `hasPaywall` to SwiftAPITester
Simplifies the common pattern in tests:
```swift
expect(array).to(haveCount(1))
let element = try XCTUnwrap(array.first)
```
Into simply:
```swift
let element = try XCTUnwrap(array.onlyElement)
```
## Changes:
- Added `PostOfferResponse`
- Added `PostOfferDecodingTests` tests
- Using the automatic deserialization by `HTTPClient` by simply changing the callback to `HTTPResponse<PostOfferResponse>.Result`
- All tests in `BackendPostOfferForSigningTests` (which are very exhaustive) pass with no changes!
Antonio Pallares