Hydrate a Kernel Manager when calling GatewayKernelManager.start_kernel with a kernel_id

[Zsailer]

Fixes #786

[codecov-commenter]

Codecov Report

Merging #788 (39df852) into main (cc8bdf4) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #788      +/-   ##
==========================================
- Coverage   70.32%   70.31%   -0.01%     
==========================================
  Files          62       62              
  Lines        7554     7555       +1     
  Branches     1248     1248              
==========================================
  Hits         5312     5312              
- Misses       1859     1860       +1     
  Partials      383      383              

Impacted Files	Coverage Δ
jupyter_server/gateway/managers.py	59.66% <66.66%> (-0.17%)	⬇️

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[kevin-bates]

LGTM - thanks @Zsailer.