Fix missing require for with_indifferent_access outside Rails

[keithmgould]

Was playing around with Servus outside of Rails and came upon this issue.

• Fix missing require 'active_support/core_ext/hash/indifferent_access' in gem/lib/servus.rb
• The gem uses with_indifferent_access in 6 places (base.rb, event.rb, validator.rb) but never explicitly requires the ActiveSupport extension that provides it
• Works in Rails because Rails eagerly loads all of ActiveSupport; breaks in plain Ruby/Rack apps with NoMethodError: undefined method 'with_indifferent_access' for
  an instance of Hash

Test plan

• Verified the single-line change adds the require alongside the existing core_ext/class/attribute require
• Run existing test suite to confirm no regressions
• Test in a non-Rails Ruby app (e.g. plain Rack) to confirm the fix resolves the NoMethodError