Updated validator contract manager and added get_collations_with_score function

[hwwhww]

Description

This PR is part of the evolution of stateless client #31

1. Updated validator_manager_contract

   • Added get_eligible_proposer function for replacing sample function
   • Added some comments and refactored
   • Removed some of shuffling cycle code and the rest will be removed after integrating with stateless collator

2. Added the easiest part of stateless_collator.py. collator.py will be replaced with more PRs.

   • Added get_collations_with_score function
   • Added get_collations_with_scores_in_range function
   • These two function will be used for CREATE_COLLATION

3. According to the above code and design, updated doc.md.

How to test

pytest sharding/tests

• Especially, sharding/tests/test_stateless_collator.py, test_validator_manager.py, and test_validator_manager_utils.py.

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