Allow passing custom enum values to RubyLLM schema

[Yuliia7-1]

Currently, we want the ability to pass dynamic enum values to RubyLLM schemas. For example, we want to be able to define a schema like this:

module Schemas
  class SummarySchema < RubyLLM::Schema
    def initialize(user_params: {})
      @user_params = user_params
    end

    array :users, description: "List of users to be created", required: false do
      object do
        string :name,
               enum: -> @user_params[:user_names],
               required: false

        array :tool_names,
              of: :string,
              enum: -> @user_params[:tool_names],
              required: true
      end
    end
  end
end

Goal:

• Enable passing custom enum values to schema fields dynamically.
• Support using lambda or proc to provide enum values at initialization.

Benefit:
This will make the schemas more flexible and reusable with dynamic input parameters.