Add /taxa/lifelist_metadata endpoint and integrate into ObservationController.life_list()

[JWCook]

Closes #511, closes #515

[codecov[bot]]

Codecov Report

Merging #512 (155001b) into main (63d97a0) will increase coverage by 0.06%. The diff coverage is 100.00%.

:exclamation: Current head 155001b differs from pull request most recent head 5bad51d. Consider uploading reports for the commit 5bad51d to get more accurate results

@@            Coverage Diff             @@
##             main     #512      +/-   ##
==========================================
+ Coverage   96.10%   96.17%   +0.06%     
==========================================
  Files          58       58              
  Lines        3288     3294       +6     
  Branches      640      643       +3     
==========================================
+ Hits         3160     3168       +8     
+ Misses         92       91       -1     
+ Partials       36       35       -1     

Impacted Files	Coverage Δ
pyinaturalist/v1/__init__.py	100.00% <ø> (ø)
...yinaturalist/controllers/observation_controller.py	91.02% <100.00%> (+0.61%)	:arrow_up:
pyinaturalist/docs/templates.py	100.00% <100.00%> (ø)
pyinaturalist/v1/observations.py	98.29% <100.00%> (+1.68%)	:arrow_up:
pyinaturalist/v1/taxa.py	93.75% <100.00%> (+0.41%)	:arrow_up:

... and 5 files with indirect coverage changes

[JWCook]

@synrg This may still need some more work, but these changes are in main now.