x-pack/filebeat/input/{cel,httpjson}: make DisableKeepAlives configurable

[efd6]

The cel and httpjson inputs both disable keep-alives unconditionally (here and here). This can lead to negative performance impacts in some cases.

We should consider allowing the user to specify this behaviour, defaulting to the current behaviour.

When this is done the input should be properly checked to ensure that bodies are always read to completion and closed to prevent resource loss. In both cases this looks to be true. Relevant code:

• cel: all HTTP requests deserialise there body through respToMap which reads the full body and closes it here.
• httpjson: all HTTP requests are made via the *httpClient.do method which closes in a defer here, and reads the full body in drainBody

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)