add an example for azure and gha usage

[AlexandreODelisle]

Description

Add examples to consume the STS service with GitHub Action and Azure.

Related Issue

Closes #10

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring / code cleanup
• CI/CD or tooling update

Changes Made

• GitHub Action inside a workflow
• Azure inside a workflow
• Azure for an application or locally using a Service Principal.

[Copilot]

Pull request overview

Adds a new EXAMPLES.md document to provide concrete end-to-end examples for exchanging OIDC/JWTs via github-sts, specifically covering Azure (Entra ID) and GitHub Actions native OIDC usage.

Changes:

• Added Azure AD (Entra ID) examples for local/CLI usage and for GitHub Actions (via azure/login).
• Added a GitHub Actions “native OIDC” workflow example (requesting an ID token from GitHub and exchanging it via github-sts).
• Documented prerequisite oidc.allowed_issuers configuration for Azure and GitHub Actions.

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