Deprecate `processors.ifname` plugin

Use Case

With the addition of the processors.snmp_lookup, the processors.ifname has become redundant.

Expected behavior

Users of processors.ifname plugin should receive a deprecation warning.

Actual behavior

No deprecation warning.

Additional info

The following config will use the same ifDescr fallback as processors.ifname when there is no ifName value on the device.

[[processors.snmp_lookup]]
  agent_tag = "agent"
  index_tag = "ifIndex"

  [[processors.snmp_lookup.tag]]
    oid = ".1.3.6.1.2.1.2.2.1.2"
    name = "ifName"

  [[processors.snmp_lookup.tag]]
    oid = ".1.3.6.1.2.1.31.1.1.1.1"
    name = "ifName"

- foo,agent=127.0.0.1,ifIndex=2 field=123
+ foo,agent=127.0.0.1,ifIndex=2,ifName=eth0 field=123