Implemented a slog handler that attached trace/correlation logs (if
available) to the log message. Also will report specific log level logs
as errors through an apm tracer.

Add documentation links and clean up comments

Implemented ability to report multiple apm errors from one log. If a user adds
multiple "reportable" error attributes to the log msg (default is "error" & "err"),
instead of trying to join the errors into one or discarding one, the apmslog
handler will report both errors.

Added ability for a user to define what slog attribute keys they want to report
as errors. Because there is no standard way in slog to attach an error to a msg
log, I wanted to add the ability for the user to decide what is and what is not
going to be reported. By default, slog attribute keys that are "error" or "err"
are reported, but with the new `WithErrorRecordAttrs(keys)` function a user
can define which keys will be reported.

Cleaned up `ApmHandler` struct and methods. Since we want the user to use the
included `NewApmHandler` function and its functional option functions, I
decided to make all Struct fields private.

Additionally added a check on if the `ApmHandler`'s `tracer` field is nill before
trying to use it. It is still possible for a user to pass in a nil tracer using
the `WithTracer` functional option.

New tests and documentation added.