Add custom AWS Region support for STS endpoint to hashicorp provider

[sdaberdaku]

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Summary

This PR introduces a configurable AWS region parameter for Vault authentication via AWS IAM in the HashiCorp provider. Previously, the airflow.providers.hashicorp._internal_client.vault_client._VaultClient._auth_aws_iam() method did not pass a region parameter to hvac.api.auth_methods.Aws.iam_login(), causing authentication failures when Vault was configured with an STS endpoint outside us-east-1.

Changes Introduced

• Added a region parameter to VaultBackend, VaultHook, and _VaultClient, allowing users to specify the AWS region for STS authentication.
• If region is not explicitly provided, it defaults to the region configured in boto3 (e.g., via environment variables or instance metadata).
• Passed the region parameter to iam_login() in hvac.
• Updated tests to validate region configuration.
• Passed the role_id parameter to iam_login() even when not using key_id and secret_id for authentication. If not passed to iam_login(), the login endpoint looks for a role bearing the name of the AMI ID of the EC2 instance that is trying to login if using the ec2 auth method, or the "friendly name" (i.e., role name or username) of the IAM principal authenticated. If the name does not match the Vault role, login fails.

Related Issue

closes: #47470

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.