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

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
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