[Return-types: Fix #1] Update `LieAlgebraOptimizer` for new return types

[eddddddy]

Context: Part of the effort to port all PennyLane functionality to the new return type system.

Description of the Change: Not much needs updating. The LieAlgebraOptimizer calls qml.execute on tapes that have only one observable. In addition, the results are already reshaped to squeeze out the extra leading dimension.

Added test file to confirm that everything works with the new return types.

[github-actions[bot]]

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[codecov[bot]]

Codecov Report

Merging #3090 (a6d9fed) into master (b025e1d) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3090   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files         273      273           
  Lines       23604    23604           
=======================================
  Hits        23530    23530           
  Misses         74       74           

Impacted Files	Coverage Δ
pennylane/optimize/lie_algebra.py	100.00% <100.00%> (ø)

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