feat: add IoT provider support and related credentials definitions

[SagiROosto]

This update introduces the IoT provider creation function and adds necessary environment variable definitions for IoT credentials in the AWS module. Additionally, the CMake configuration is updated to include the new IoT credentials source file.

[yuvalDOosto]

Good job, added some comments, I have to look at it again due to flb_aws_credential_iot.c length but the current comments are a good start

[Copilot]

Pull Request Overview

This PR introduces AWS IoT credentials provider support to Fluent Bit, enabling authentication using IoT device certificates. The implementation adds a new credentials provider that uses IoT-specific environment variables and TLS certificate authentication to obtain temporary AWS credentials from an IoT credentials endpoint.

Key changes:

• Added complete IoT credentials provider implementation with TLS certificate support
• Integrated IoT provider into the standard AWS credentials chain
• Added environment variable definitions for IoT configuration parameters

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/aws/flb_aws_credentials_iot.c	New IoT credentials provider implementation with certificate-based authentication
src/aws/flb_aws_credentials.c	Integration of IoT provider into standard credentials chain
src/aws/CMakeLists.txt	Added new IoT credentials source file to build
include/fluent-bit/flb_aws_credentials.h	Added IoT environment variable definitions and function declaration

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[SagiROosto]

since the PR to original repo is from the master branch and still in progress, we'll wait for it to be merged first fluent#10824