feature: add dns-01 validation option

[klowdo]

Hi!

Added dns-01 validation. im using this i Azure container apps to add certs with the container app env as certificate source. Very new to this so happy for all feedback.

and in order to get this working behind a reverse proxy im adding this and binding the dns with cert in other code

var lettuceKesterConfig =  builder.Services.FirstOrDefault(
                x => x.ServiceType.FullName == "LettuceEncrypt.Internal.KestrelOptionsSetup");
if (lettuceKesterConfig != null)
    builder.Services.Remove(lettuceKesterConfig);

[natemcmaster]

This looks great @klowdo ! Thanks so much for sending this PR. This has been a long-time requested feature for this project.

I'm going to merge your code, but make 2 tweaks after I do. I'm going to move all public interfaces and types out of the LettuceEncrypt.Internal.* namespace.

Really nice work though! Thanks for the contribution 👍🏻