[Refactoring] Consolidate common HTTP settings

[ycombinator]

We have a few HTTP clients spread across the Beats codebase. There is a common set of configuration options that apply to any HTTP client, e.g. host, timeouts, SSL-related configuration, etc. It would be nice to consolidate such common options into a struct somewhere under libbeat/common/transport perhaps and then use it in the various HTTP clients. This would ensure that all clients support the same common configuration optinos and users can specify them using the same setting names.

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[botelastic]

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