Add SARS tdlearning back to lib

[jeremiahpslewis]

• One of a couple pull requests on the way to getting plain vanilla q-learning working
• Drops optimiser from TabularApproximator, too many unforced errors with it specified
• Approximator becomes FluxModelApproximator for clarity
• TabularApproximator and FluxModelApproximator are independent subtypes of AbstractLearner

PR Checklist

• Update NEWS.md?
• Unit tests for all structs / functions?
• Integration and correctness tests using a simple env?
• PR Review?
• Add or update documentation?
• Write docstrings for new methods?

[codecov]

Codecov Report

Attention: Patch coverage is 80.35714% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 57.50%. Comparing base (55f60b0) to head (a0330b8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1050      +/-   ##
==========================================
+ Coverage   55.68%   57.50%   +1.82%     
==========================================
  Files          69       71       +2     
  Lines        2726     2751      +25     
==========================================
+ Hits         1518     1582      +64     
+ Misses       1208     1169      -39     

Files	Coverage Δ
...ementLearningCore/src/policies/agent/agent_base.jl	71.42% <ø> (ø)
...e/src/policies/learners/flux_model_approximator.jl	100.00% <100.00%> (ø)
...Core/src/policies/learners/tabular_approximator.jl	100.00% <100.00%> (ø)
...ementLearningFarm/src/ReinforcementLearningFarm.jl	100.00% <ø> (ø)
...arningCore/src/policies/learners/target_network.jl	95.83% <80.00%> (ø)
...ntLearningCore/src/policies/learners/td_learner.jl	91.30% <91.30%> (ø)
...rcementLearningCore/src/policies/q_based_policy.jl	71.42% <66.66%> (+71.42%)	⬆️
.../src/policies/explorers/epsilon_greedy_explorer.jl	23.45% <14.28%> (+23.45%)	⬆️

... and 3 files with indirect coverage changes

[HenriDeh]

LGTM. Good idea naming the FluxApproximator. One day this could be moving to a Flux extension.