Added Proxy support for action types which are integrating with external services

[YulNaumenko]

Added proxy settings for defining proxyUrl and proxyHeaders under kibana.yml xpack.actions configuration.
The decision was to use HTTP/HTTPS proxy, not SOCKS, because it is the most commonly used and all action types libraries support it.
For enabling self-signed certificates added rejectUnauthorized: false for all external requests (axios, nodemailer, slack).

1. For email action type is used Nodemailer library and proxy configuration is built-in https://nodemailer.com/smtp/proxies/#1-using-http-proxy
2. For slack action type is used node-slack-sdk, which does not have a built-in proxy, but allows to configure it using https-proxy-agent
3. For the rest of action types axios library is used to make requests to external services. This library has built-in proxy configuration for HTTPS, but for enabling self-signed certificates we need to setting rejectUnauthorized: false which is configurable only under the agent options. So implemented proxy support using httpsAgent for HTTPS proxy config and use native proxy implementation for the HTTP proxy.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[kibanamachine]

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• 💔 Build #63821 failed 0b2a649519c505bf3e489e8c86b1624cac2c2f90
• 💔 Build #63769 failed f29a6c4fea925ef185f7c82e33d92a91d88347f7
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