Fix/epoll iocp headers

[regyssilveira]

PR: Fix HTTP headers and response body mapping on async/high-performance providers

Description

This PR resolves the issue where custom/standard HTTP headers and response contents (such as JSON generated by middlewares) were not correctly delivered or sent when using asynchronous and high-performance providers (epoll, IOCP, and HttpSys).

Key Fixes:

1. HTTP Headers Mapping:

   • Added PopulateHeaders method to the IHorseRawRequest interface.
   • Implemented PopulateHeaders in TEpollRawRequest, TIocpRawRequest, and THttpSysRawRequest (extracting headers dynamically according to the requested NameValueSeparator).
   • Integrated THorseCoreParamHeader.GetHeadersList to delegate to PopulateHeaders if the web request is a TInterfacedWebRequest.

2. Response Content Delivery:

   • Implemented Content and ContentStream properties on the Delphi TInterfacedWebResponse adapter class.
   • Updated the SendResponse routine in both epoll and IOCP providers to extract and output data from the adapter's Content/ContentStream properties when populated.

3. Compiler and Version Compatibility:

   • Fixed compilation on older versions of Delphi (e.g. Delphi 10 Seattle) by shielding GetRawContent inside appropriate version conditionals (CompilerVersion >= 32.0).
   • Replaced TEncoding.UTF8.GetString with standard SetString (zero-allocation and cross-compiler compatible) in the FPC/epoll provider code.

4. Integration Testing:

   • Added unit test cases for custom HTTP headers on Windows (Tests.Api.Console.pas).
   • Enabled -dHORSE_PROVIDER_EPOLL on Lazarus integration tests within the Docker container.

Validation and Tests

• Delphi/Windows: Verified with all Delphi versions (10 Seattle up to 13 Florence) under both IOCP and HttpSys providers. All tests passed.
• FPC/Linux: Verified with the epoll loop inside the Lazarus docker container. All integration tests succeeded.