Add implementation of ordindary DNS provider for query plain old DNS records

[prologic]

Closes #185

[dnephin]

Thank you for the PR! I'd like to better understand your use case. Are you using this library directly, or are you hoping for this support in Consul (or some other HashiCorp product) ?

[prologic]

Thank you for the PR! I'd like to better understand your use case. Are you using this library directly, or are you hoping for this support in Consul (or some other HashiCorp product) ?

I'm hoping this will get into go-discover itself so that ordinary DNS-based Service Discovery, such as that found in any non-public cloud infrastructure can be used in Hashicorp products ​such as Consul and Vault.

Thank you for the consideration 🙇‍♂

[hashicorp-cla]

All committers have signed the CLA.

[prologic]

NB: I've rebased this on #200 for the time being. Fixed the tests and blakiness and this PR should be ready to go!