Support node18

[dlongley]

This is being tested in a number of projects to determine if the changes are sufficient.

[dlongley]

Looks like we can't get to full node 18 support yet because we can't set cookie headers in requests. The issue of cookies w/ undici fetch is under discussion over here: nodejs/undici#1262

[codecov-commenter]

Codecov Report

Merging #26 (6270975) into main (2a47f7b) will decrease coverage by 12.39%.
The diff coverage is 56.33%.

@@             Coverage Diff             @@
##             main      #26       +/-   ##
===========================================
- Coverage   95.27%   82.87%   -12.40%     
===========================================
  Files           2        3        +1     
  Lines         148      216       +68     
===========================================
+ Hits          141      179       +38     
- Misses          7       37       +30     

Impacted Files	Coverage Δ
lib/agentCompatibility.js	49.05% <49.05%> (ø)
lib/httpClient.js	93.46% <77.77%> (-1.47%)	⬇️

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