[ResponseOps] meta - improve proxy support

[pmuellr]

There are a number of issues with our proxy support that we should address. This is a meta issue pointing to them. These issues should get us to the point of having proxy support working across a larger set of scenarios than it does today, and ensure they will work into the future (adding tests):

• not enough proxy tests - [actions] add extensive proxy + email jest tests #102935
• more documentation for proxy support - [Connectors] Document the recommended architecture for connectors working with proxy servers #135930
• add proxy option support to hpagent - Add support for proxyRequestOptions delvedor/hpagent#72
• switch proxy implementation to hpagent - [responseOps] switch proxy implementation from http(s)-proxy-agent to hpagent #127825
• support for proxy authentication - [Connectors] Implement support for Proxy-Authorization. #106770
• support for "custom settings" for proxies (eg, certificate authority) - Add ability to define Proxy configuration for Kibana in ECE User Settings #132187, verificationMode ignored when proxy is used for Alerting #130698
• fix proxy headers issues - [ResponseOps] better documentation on proxyHeaders config #124250
• global rejectUnauthorized setting not honored for email - [ResponseOps] global rejectUnauthorized setting not honored for email #141220

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[pmuellr]

Re-looked at this to see if there were any updates since my last check.

There's one update - the third bullet "add proxy support to hpagent" is complete, and was the blocker for us to continue work in Kibana. There's just one little thing to fix.

We want at least v1.2.0 of hpagent. Currently, we don't have a direct dependency on hpagent in our package.json. However, we get it for free via the elasticsearch package. So it is available to us, and yarn is getting the most recent one:

https://github.com/elastic/kibana/blob/7c71786ff9537995aa79256c34a7e47c37ea3993/yarn.lock#L17113-L17116

We will need to add a dependency to the 1.2.0 version in Kibana's package.json, to make everything kosher.

We can do that in the first PR that makes use of hpagent in our proxy support.