[processor/resourcedetection]: add "cname" and "lookup" hostname sources

[pmcollins]

[processor/resourcedetection]: add "cname" and "lookup" hostname sources

The existing "dns" hostname source uses the Showmax/go-fqdn library, whose FqdnHostname()
function tries to get the fully qualified domain name of the current host by

1. Reading the hosts file and looking for a match of the os.Hostname and a corresponding fqdn
   on that line. If that fails, then it falls back to:
2. Getting the canonical name via net.LookupCNAME. If that fails, then it falls back to:
3. Getting the current IP address, and doing a reverse DNS lookup of that address

A problem that least one Collector user encountered while running on Windows in Azure,
is that the call to net.LookupCNAME just returns the simple hostname, which
then gets returned by FqdnHostname(), even though on a Linux host, deployed in the same
environment, net.LookupCNAME returns a fully qualified domain name.

Instead of trying to work around what might be an issue with net.LookupCNAME by modifying
the Showmax/go-fqdn code, which has hard-coded fallback logic, and changing behavior for
existing users, this change proposes creating two new hostname sources, "cname", and "lookup".
The "cname" source corresponds to a call to net.LookupCNAME, and the "lookup" source corresponds
to a call to net.LookupHost and then a call to net.LookupAddr for each ip address it returns.
The "lookup" source produces the same result on Windows as net.LookupCNAME does on Linux and would
solve the problem of the Azure user in question. In addition, users can then build the fallback logic that they prefer.
Eventually, we could also add a "hostfile" hostname source that uses the same logic as the
corresponding part of the Showmax/"dns" implementation, so that users could replace the "dns"
hostname source with the equivalent three sources, explicitly indicating the fallback logic rather
than using Showmax's hard-coded logic:

processors:
  resourcedetection:
    detectors: ["system"]
    system:
      hostname_sources: ["hostsfile", "cname", "lookup"]

At that point, we could deprecate the "dns" source or make it effectively an alias for ["hostsfile",
"cname", "lookup"] and remove the dependency.

For now, the customer that would benefit from this change, however, will just use the "lookup" source on
their Windows hosts, as "cname" on Windows doesn't produce a useful result.

[dmitryax]

At that point, we could deprecate the "dns" source or make it effectively an alias for ["hostsfile",
"cname", "lookup"] and remove the dependency.

I like this idea. We should do that and eventually remove Showmax/go-fqdn dependency. Could you please create a follow up issue?

[dmitryax]

Can you add some test coverage?

[pmcollins]

Can you add some test coverage?

Sure -- I've added some.

[pmcollins]

Created follow-up issue #10138