Add support for Rails 8.1

[bassemawhoob]

Motivation / Background

This pull request adds support ActiveRecord 8.1 and improves compatibility by introducing a NullPool implementation for connection pooling. It also reorganizes some internal requires for better code structure.

Detail

• Updated the gemspec to allow ActiveRecord versions up to 8.1, and added a new test gemfile for ActiveRecord 8.1 support.
• Added a new Tableless::NullPool class that inherits from ActiveRecord::ConnectionAdapters::NullPool and implements the with_pool_transaction_isolation_level method for compatibility with recent ActiveRecord versions.
• Updated Tableless::DummyAdapter to initialize and use the new NullPool for connection pooling.

Fixes #6

[bassemawhoob]

@jonian

[jonian]

Thank you for your contribution @bassemawhoob !