CSR flows for IoT Hub

[maximsemenov80]

This pull request introduces support for Certificate Signing Request (CSR) flows during both Device Provisioning Service (DPS) provisioning and after connecting to IoT Hub in the Linux demo configuration. It also updates related documentation, configuration options, and enables an additional elliptic curve in mbedTLS. The CMake presets are updated to add a Linux build configuration.

CSR Feature Enhancements:

• Added configuration options in demo_config.h to enable CSR during DPS provisioning (democonfigENABLE_DPS_CSR) and/or after IoT Hub connection (democonfigENABLE_IOT_HUB_CSR). Shared macros for CSR data and private key are now used by both flows, and a unique request ID is supported for IoT Hub CSR.
• Updated documentation in README.md to describe the new CSR flows, required parameters, and example macro definitions for both DPS and IoT Hub CSR scenarios. [1] [2] [3]
• Deprecated the old democonfigCERTIFICATE_SIGNING_REQUEST macro in favor of democonfigCERTIFICATE_SIGNING_REQUEST_DATA for clarity and consistency. [1] [2] [3]

Build System Improvements:

• Added a new Linux build preset to CMakePresets.json for easier configuration and building on PC/Linux platforms. [1] [2]

Security/Compatibility Updates:

• Enabled the SECP384R1 elliptic curve in mbedtls_config.h to support a wider range of X.509 certificates.
• Updated the Azure IoT middleware submodule to the latest commit for bug fixes and improvements.

Other:

• Clarified placeholder values in demo_config.h for device symmetric key.