feat(toolkit-lib)!: support custom https.Agent for SDK requests

[mrgrain]

Fixes #398
Depends on #532

Previously automatically configured the SDK to pass all requests through an instance of ProxyAgent from the third-party proxy-agent package. A user could configure its proxy settings, but by default it would act as a pass through. proxy-agent is a hefty dependency (primarily due to its support for pac files) that not everyone might want to use. This is the standard interface in Node.js and we are aligning on it.

Note that with this we are also removing built-in (but not advertised) support for managing proxy configuration through env vars like HTTP_PROXY or HTTPS_PROXY. This functionality was provided by proxy-agent and the easiest way to get it back is by configuring proxy-agent again.

BREAKING CHANGE: Dependency on proxy-agent has been removed, this also removes the support of configuring a proxy through like HTTPS_PROXY and AWS_CA_BUNDLE. The type of sdkConfig.httpOptions has changed to now optionally take a single agent: https.Agent property instead of the proxy settings. The restore the previous behavior, pass an instance of ProxyAgent to sdkConfig.httpOptions: { agent: new ProxyAgent({ ca: oldHttpOptions.caBundlePath, getProxyForUrl: () => oldHttpOptions.proxyAddress }) }

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.88%. Comparing base (70128c0) to head (8c4f26f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #533      +/-   ##
==========================================
- Coverage   79.01%   78.88%   -0.13%     
==========================================
  Files          47       47              
  Lines        7032     7032              
  Branches      786      781       -5     
==========================================
- Hits         5556     5547       -9     
- Misses       1458     1467       +9     
  Partials       18       18              

Flag	Coverage Δ
suite.unit	78.88% <ø> (-0.13%)	⬇️

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