Client classes for Servers

[madhur-tandon]

Fixes #40

I could appreciate a pair session for this task since the ebonite reference is kinda overwhelming -- there are many things I am sure of which we could eliminate. CC: @mike0sv

[codecov]

Codecov Report

Merging #185 (f77f948) into main (8b5dace) will increase coverage by 2.03%.
The diff coverage is 97.08%.

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
+ Coverage   87.48%   89.52%   +2.03%     
==========================================
  Files          74       74              
  Lines        4812     4896      +84     
==========================================
+ Hits         4210     4383     +173     
+ Misses        602      513      -89     

Impacted Files	Coverage Δ
mlem/runtime/client/base.py	95.23% <95.00%> (-4.77%)	⬇️
mlem/contrib/fastapi.py	95.16% <100.00%> (+0.61%)	⬆️
mlem/contrib/lightgbm.py	95.77% <100.00%> (+0.18%)	⬆️
mlem/contrib/numpy.py	94.94% <100.00%> (+0.05%)	⬆️
mlem/contrib/xgboost.py	97.56% <100.00%> (+0.06%)	⬆️
mlem/core/dataset_type.py	96.31% <100.00%> (+9.74%)	⬆️
mlem/core/meta_io.py	94.96% <0.00%> (+0.62%)	⬆️
mlem/core/objects.py	91.11% <0.00%> (+3.70%)	⬆️
mlem/contrib/docker/base.py	86.18% <0.00%> (+4.41%)	⬆️
mlem/api/utils.py	88.88% <0.00%> (+4.44%)	⬆️
... and 5 more

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[madhur-tandon]

@mike0sv we can discuss writing tests a bit. The endpoint /interface.json works, but parse_obj_as(InterfaceDescriptor, resp.json()) seems to fail due to

pydantic.error_wrappers.ValidationError: 1 validation error for ParsingModel[InterfaceDescriptor]
__root__
  value is not a valid dict (type=type_error.dict)

Moreover, since we are using our own client instead of FastAPI's TestClient, we need to run the server (as a fixture) within a thread or perhaps a different process.
We can fix this today and wrap this PR if it's okay by a bit of discussion.

[madhur-tandon]

@mike0sv ready to merge!