[TCP Input] Support Proxy Protocol

**Describe the enhancement:**
Have Elastic Agent support proxy protocol when behind a L4 load balancer like haproxy or nginx.

**Describe a specific use case for the enhancement or feature:**
Preserve the original IP address of the sender without having it present in the log itself.

**What is the definition of done?**
Having some sort of configurable option that will globally apply the proxy protocol parsing for all TCP related inputs.
Maybe even have it per integration as it is not necessary for some.

Currently the only option is to parse the message itself per integration using the `processors` option to extract the proxy protocol appended header.

Start by tracing the TCP-related inputs and the existing processors option used to parse proxy protocol headers. Determine where a global or per-integration configuration can apply before input processing. Done means a configurable proxy-protocol parser preserves the sender's original IP for supported TCP inputs without requiring integration-specific message parsing.

Written by the indexing model from the issue text.