Add support for api key

[wslulciuc]

This PR adds API key support to our client. For more details see MarquezProject/marquez#817. This PR also fixes a bug to the HTTP backend when using the default marquez URL.

[codecov]

Codecov Report

Merging #119 into main will increase coverage by 2.45%.
The diff coverage is 56.06%.

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   38.16%   40.62%   +2.45%     
==========================================
  Files          21       22       +1     
  Lines         862      901      +39     
==========================================
+ Hits          329      366      +37     
- Misses        533      535       +2     

Impacted Files	Coverage Δ
marquez_client/backend.py	0.00% <ø> (ø)
marquez_client/client.py	0.00% <0.00%> (ø)
marquez_client/clients.py	0.00% <0.00%> (ø)
marquez_client/constants.py	0.00% <0.00%> (ø)
marquez_client/http_backend.py	0.00% <0.00%> (ø)
marquez_client/utils.py	0.00% <0.00%> (ø)
tests/test_marquez_clients.py	100.00% <100.00%> (ø)

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