Closes#695 and finishes [CF-195].
Depends on ~~#1431, #1432, #1433, #1437~~.
## Goals:
- [x] Handle all requests / response / deserialization errors in `HTTPClient`. Clients of `HTTPClient` will only have to handle the "happy" path: #1431
- [x] `HTTPClient` shouldn't return `Result.success` with failed responses, forcing clients to verify the response is actually a failure: https://github.com/RevenueCat/purchases-ios/pull/1431
- [x] Abstract error response deserialization / error creation: #1427
- [x] Abstract attribute error parsing: #1427
- [x] Move response deserialization to `HTTPClient` based on a `ResponseType: Decodable` type, so the completion block will simply return a `Result<HTTPResponse<ResponseType>, Error>`
- [x] `ETagManager` should store response `Data` instead of a deserialized `[String: Any]`
- [x] Improved error types in `HTTPClient` to fix tests: #1437
- [x] Dealt with non-backwards compatible `ETagManager: #1438
## Changes:
- Replaced `HTTPResponse`'s body from `[String: Any]` to a generic `HTTPResponseBody`.
- Created `HTTPResponseBody` to abstract `Decodable` and provide some default implementations for types like `Data,` `[String: Any]` (for backwards compatibility to types that aren't `Decodable` yet), and `Decodable` itself.
- New `HTTPResponse.Result` typealias (`Result<HTTPResponse<HTTPResponseBody>, Error>`) used everywhere. This will allow replacing `Error` with a more specific `Error` so we can forward known typed errors, and make sure that we don't end up with the wrong error type, or with a very complex error hierarchy and the details buried in `underlyingError`.
- Each layer (only a few for now) has its own error type: `NetworkError`, `BackendError`, `OfferingsManager.Error`.
- `HTTPClient` for example has to produce `NetworkError`, operations produce `BackendError`
- The parent `BackendError` can have specific errors like `.missingAppUserID`, but also be simply a child error `case networkError(NetworkError)`
- All of these conform to a `ErrorCodeConvertible`, so there is a single point of code that converts from simple and readable errors (like `BackendError.emptySubscriberAttributes`, `.unexpectedBackendResponse(.loginResponseDecoding)`) into errors with all the context using `ErrorUtils`
- Converted `DNSError` into `NetworkError.dnsError`. Its functionality remains unchanged.
- Removed `Backend.RCSuccessfullySyncedKey` and `ErrorDetails.finishableKey` in favor of tested properties on `NetworkError`
### Leftovers
There's a few things I've decided to not finish for now:
- [ ] `CustomerInfo` still does't conform to `Decodable` (manual deserialization is still supported by the current system): #1496
- `SubscriberAttributeDict` could also be `Codable`
- [x] Replace `OfferingsFactory` with `Decodable`: #1435
[CF-195]: https://revenuecats.atlassian.net/browse/CF-195?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
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