simplify reward structure; add reward_exact_answer

[andytwigg]

Description

simplify reward structure
comment out check_answer due to overlap with check_numbers
add reward_exact_answer

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/trainers/post_train/rl/train_rl.py	16.66%	5 Missing ⚠️
src/maxtext/trainers/post_train/rl/utils_rl.py	25.00%	3 Missing ⚠️

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[richjames0]

lgtm