[Feature] - Support v4 and v6 PTR records by annotation

[jtackaberry]

For standalone LBs, Vultr allows configuring PTR records for both IPv4 and IPv6. For LBs provisioned and managed by vultr-cloud-controller-manager, this capability should be surfaced via annotations.

For v4, this is a simple scalar value.

For v6, the console allows creating multiple addresses. As I'm new to Vultr, I'm not clear on the use case yet (because while creating PTRs for IPv6 addresses other than the one presented does properly resolve, it doesn't cause the LB to bind to this address), but a format similar to firewall-rules would make sense, where you have a semicolon delimited list defining multiple addresses. Each element of the list can be in the form <64-bit suffix>,<hostname> or, for the LB's primary assigned IP, simply <hostname>.

Thanks!