feat: make dynamic bucket region detection optional

[sasanyasari]

Overview

Add support for disabling dynamic bucket region detection via the HELM_S3_DYNAMIC_REGION environment variable.

Problem

The current implementation of DynamicBucketRegion always performs automatic bucket region detection by making HEAD requests to Amazon S3, which causes several issues:

1. Breaks plugin in restricted environments: Environments with limited internet access or strict egress filtering cannot make the required HEAD requests to s3.amazonaws.com
2. Security/privacy concerns: Internal bucket names are leaked to Amazon S3 even when using alternative S3-compatible storage solutions
3. Ignores user configuration: The HEAD requests always go to Amazon S3 (s3.amazonaws.com) regardless of custom endpoints configured via AWS_ENDPOINT or AWS profiles

Solution

Introduce HELM_S3_DYNAMIC_REGION environment variable:

• Default: true (enabled) - maintains backward compatibility
• Set to false: Disables dynamic region detection, relies on standard AWS region configuration

Changes

• Added ConditionalDynamicBucketRegion() function that respects the new environment variable
• Updated all helm-s3 commands to use the conditional function
• Added comprehensive tests covering both enabled and disabled scenarios
• Updated documentation in README.md and website

Usage

To disable dynamic bucket region detection:

export HELM_S3_DYNAMIC_REGION=false

This is particularly useful for:

• Environments with restricted internet access
• Alternative S3-compatible storage (MinIO, Ceph, etc.)
• Security-sensitive environments where bucket names should not be exposed to AWS
• CI/CD pipelines requiring predictable region behavior

Testing

All existing tests pass, and new tests verify:

• Default behavior (dynamic region enabled)
• Explicitly enabled dynamic region detection
• Disabled dynamic region detection

Backward Compatibility

✅ Fully backward compatible - existing users will see no change in behavior unless they explicitly set HELM_S3_DYNAMIC_REGION=false.

Fixes #220