[Feature Request]: support dns-persist in our manual plugin

for reference, the current spec for dns-persist can be found here

What problem does this feature solve or what does it enhance?

• helps people access features soon to be offered by let's encrypt
• users needn't configure their server with DNS API credentials which can pose a broad security risk
• gives certbot users a somewhat sane solution to DNS challenges and wildcard certificates agnostic to the DNS provider

Proposed Solution

in very broad strokes that need further fleshing out, i suggest we

1. sync up with let's encrypt on timelines and exactly what version of the spec to implement. i think it'd be ideal if our support for dns-persist was released around the same time as them
2. implement support for the challenge in our manual plugin allowing users to perform the challenge without giving certbot any DNS credentials
3. polish up the documentation and UX surrounding this feature