Add named Flipper instances

[jnunemaker]

Summary

• add named Flipper configurations with stable Flipper.named(:cross_app) and Flipper.cross_app access
• isolate adapter stacks, groups, feature expressions, thread-local DSLs, Rails memoization, preload behavior, and test state per instance
• support optional per-instance Flipper Cloud credentials and webhook routes without changing default Cloud configuration
• document Active Record table prefixes and per-prefix schema updates for independent local stores

Motivation

Applications with separate product-specific Flipper Cloud projects sometimes also need one shared cross-product project. Named instances allow both configurations to coexist in each process while preserving the existing top-level Flipper instance unchanged.

Testing

• bundle exec rspec spec --exclude-pattern 'spec/flipper/{adapters/**/*_spec.rb,cli_spec.rb}' --seed 40111 (1,634 examples)
• bundle exec rake test TEST=test/adapters/active_record_test.rb (36 runs, 938 assertions)
• Rails generator tests (14 runs, 111 assertions)
• focused named Cloud, middleware, UI, API, test-helper, concurrency, and configuration lifecycle coverage