Allow to override https agent

[AnatolyRugalev]

It is not currently possible to set custom https agent, although documentation of StreamChat constructor says otherwise:

   * @param {httpsAgent} [options.httpsAgent] - custom httpsAgent, in node it's default to https.agent()

[github-actions]

Size Change: +23 B (0%)

Total Size: 218 kB

Filename	Size	Change
dist/browser.es.js	47.3 kB	+4 B (0%)
dist/browser.full-bundle.min.js	27.4 kB	+6 B (0%)
dist/browser.js	47.8 kB	+5 B (0%)
dist/index.es.js	47.3 kB	+4 B (0%)
dist/index.js	47.9 kB	+4 B (0%)

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