Remove OTEL_TARGET_EXE and auto discover processes

[edeNFed]

This issue is for removing the OTEL_TARGET_EXE env variable and implementing process discovery instead.
Implementing this will make integration with OpenTelemetry Operator easier and overall provide a better experience to users (one less argument to specify).

I suggest splitting this into two phases:
phase 1: Apply automatic instrumentation to the first Go process found (there is already logic to detect Go processes at analyze.go
phase 2: Apply automatic instrumentation to any Go process found (multi-process support)