incusd/server/device/nic_routed: Added host_tables

[AbhinavTiruvee]

We closed our old PR (#1964) and are opening a new one due to it being pretty outdated compared to the codebase. We added our changes to nic_routed.go and wanted to know where else we need to edit. We assume the documentation we need to add would be through gendoc and we would mark the ipv4.host_table and ipv6.host_table as deprecated there.

Issue #1559

[stgraber]

A few small changes seem needed:

• I'd split the addition of the new logic with marking the old options as deprecated
• The commit with the deprecation notice should also change the doc string (gendoc)
• You need to add a commit (typically titled doc: Update configs) with the result of make update-metadata
• As this adds new config keys, an API extension should be added, this is done by editing both doc/api-extensions.md and internal/version/api.go, in this case, the string may be something like instance_nic_routed_host_tables

[avipatel805]

We seem to be passing all test cases except for a spellcheck on "ipv." Is this something we should change, or is it okay?