#1 do add x-possystem-id header to all requests in howtos/lib

[turtletramp]

This pull request introduces support for specifying a POS System ID when initializing and using the ftPosAPI client, improving both development and production configuration. The changes ensure that the POS System ID is read from the environment, passed through the API, and included in HTTP headers, with clear warnings and documentation for proper usage.

Configuration and environment variable handling:

• Added support for reading the FISKALTRUST_POS_SYSTEM_ID environment variable in Utils.cs, with fallback and warnings for development/testing if not set, and validation of its format.
• Updated the documentation in README.MD to explain the use and requirements of the FISKALTRUST_POS_SYSTEM_ID environment variable for production and development.

API and client initialization:

• Modified the Init method in ftPosAPI to accept a posSystemID parameter, store it, and document its usage, including guidance for production and sandbox environments.
• Updated all calls to ftPosAPI.Init to pass the POS System ID, including test initialization and utility functions. [1] [2]

HTTP request handling:

• Added the x-possystem-id header to all outgoing requests from ftPosAPI to include the POS System ID in API calls.