Fixes [CSDK-634].
## Changes:
- Created `HTTPResponseValidationResult`
- Added `HTTPResponseValidationResult` and `HTTPClient.ResponseHeaders`
to `HTTPResponse`
- Added new `ErrorCode.signatureVerificationFailed`
- Extracted `SigningType` to create `MockSigning` to test `HTTPClient`
without bothering with signatures
- Refactored `ETagManager` to use `HTTPResponse`, making it easier to
pass through response headers and verification results
- Stored `HTTPResponseValidationResult` in `ETagManager` cache
- `HTTPClient` handles signatures from response based on
`SystemInfo.responseVerificationLevel`
- Renamed `HTTPRequest.createIntegrityEnforcedRequestRequest` to
`HTTPRequest.createWithResponseVerification`
- Added lots of tests for this new behavior in `HTTPClient`
- Added lots of new tests for `ETagManager`
[CSDK-634]:
https://revenuecats.atlassian.net/browse/CSDK-634?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
This breaks signature validation in #2267 because `PurchasesDiagnostics`
uses that endpoint with and without signature validation, so the first
time it makes a request, it stores the cached response without a
validated signature.
The best solution is to simply not send etags for this endpoint, since
it's used precisely to make sure that requests can be made with no
errors, and we wouldn't want false positives.
`DNSChecker.errorWithBlockedHostFromError` already checks
`isBlockedAPIError` when constructing this error, so we can simplify
this to avoid calling `DNSChecker.resolvedHost` twice.
NachoSoto