Support for multiple S3 backends

[enzodjabali]

Add support for multiple S3 backends with environment-based configuration.

This PR introduces the ability to configure and manage multiple S3 backends within a single instance of s3manager. Users can define multiple S3 instances via environment variables prefixed with numeric identifiers (e.g. 1_NAME, 1_ACCESS_KEY_ID, 2_NAME, etc.), enabling dynamic management of multiple S3-compatible storage providers or accounts.

Key changes include:

• A new MultiS3Manager to handle multiple S3 clients and switching between them.
• Parsing of numbered environment variables to load multiple S3 instance configurations.
• Updated HTTP handlers and UI templates to support instance switching and display buckets across configured S3 backends.
• Backward compatibility with existing single-instance configuration.
• Example environment setup added in docker-compose.yml showcasing configuration of two S3 instances.

Cheers ;)

[cloudlena]

Amazing! Many thanks, @enzodjabali!