add idle_connection_timeout to HTTPTransportSettings

[leehinman]

What does this PR do?

Add idle_connection_timeout to HTTPTransportSettings

Why is it important?

Will allow users of the library that use

	Transport httpcommon.HTTPTransportSettings `config:",inline"`

in their config objects to set the keep alive setting of IdleConnTimeout by setting idle_connection_timeout

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
  ~~- [ ] I have added an entry in CHANGELOG.md ~~

Related issues

• Relates Allow queue configuration to be specified under the output type beats#35615

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 66a0ecb

[ycombinator]

Code and test changes LGTM. Will any user-facing documentation (probably not in this repo) need to be updated to mention the newly-available idle_connection_timeout setting under the ES output?

[leehinman]

Will any user-facing documentation (probably not in this repo) need to be updated to mention the newly-available idle_connection_timeout setting under the ES output?

Yep, that will be part of the PR in the beats repo.