Move server side code to emmet-api package and parallel requests

[munrojm]

This PR moves all server side API code to the new emmet-api package. It also implements parallel requests for select query parameters, including those involved in data pagination.

[codecov-commenter]

Codecov Report

Merging #475 (3e693cd) into main (ed581fd) will increase coverage by 0.18%.
The diff coverage is 91.60%.

@@            Coverage Diff             @@
##             main     #475      +/-   ##
==========================================
+ Coverage   93.12%   93.31%   +0.18%     
==========================================
  Files          68       34      -34     
  Lines        2227     1271     -956     
==========================================
- Hits         2074     1186     -888     
+ Misses        153       85      -68     

Impacted Files	Coverage Δ
src/mp_api/__init__.py	100.00% <ø> (+9.09%)	⬆️
src/mp_api/routes/bonds.py	95.83% <ø> (ø)
src/mp_api/routes/electrodes.py	96.42% <ø> (ø)
src/mp_api/routes/magnetism.py	96.29% <ø> (ø)
src/mp_api/routes/oxidation_states.py	100.00% <ø> (ø)
src/mp_api/routes/piezo.py	93.75% <ø> (ø)
src/mp_api/routes/provenance.py	100.00% <ø> (ø)
src/mp_api/routes/charge_density.py	92.59% <66.66%> (ø)
src/mp_api/core/client.py	89.89% <86.80%> (-0.51%)	⬇️
src/mp_api/routes/robocrys.py	92.30% <92.30%> (ø)
... and 28 more

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