[typescript] Adding Custom Agent Support for fetch call

Created by: 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 (closed)

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