[typescript] Adding Custom Agent Support for fetch call

[davidgamero]

Adding agent: http.Agent | https.Agent | undefined instance variable on the RequestContext class as well as the following methods:

setAgent(agent: http.Agent | https.Agent)
getAgent() : http.Agent | https.Agent | undefined

Then, the isomorphic fetch implementation would call getAgent() to pass the value into the agent option on the fetch call in isomorphic-fetch.ts

Implements #11394

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@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov

[macjohnny]

Thanks for the contribution, looks good.
Can you please revert the deleted files and update the samples?

[davidgamero]

yep merging master back in now

[macjohnny]

@TiFu are you fine with this change?

[davidgamero]

ended up excluding the web fetch since the lib doesn't list agent support, so it's just for the nodejs

[TiFu]

LGTM! Thank you for the PR! :)