Temporarily copy external dns plan and registry code

[mikenairn]

closes #68

Import external dns plan package and txt registry

Moves the external-dns plan package into this repo along with the txt registry.

To ease development, code reviews, and make it easier for folks to get up to speed with plan/registry code, we will initially just work off a copy of the relevant external-dns code in this repo. When we have a solution that works for us we will look into how we can submit that back to external-dns.

Code is copied, unmodified where possible, from the v0.14.1 version of external-dns. https://github.com/kubernetes-sigs/external-dns/tree/v0.14.1

registry: Remove old txt record creation

Remove the creation of the old format txt records. It adds another extra record for every endpoint we are managing and it's unnecessary in our case since we don't have any old style records in existence that need migrated.

Update external-dns go module to v0.14.1

Changes to various go modules versions, but nothing major. Additional config required around batching requests for AWS provider, added external-dns default values for this.

Notes:

• There are no functional changes here, code has only moved, test passing should be a good enough review.
• All code in internal/extrnal-dns should be considered separate from the main dns-operator code and we should treat it as such. Logging and testing for example should be kept consistent with how external-dns approaches things and we should not import our own code outside of internal/external-dns into here.
• external-dns uses a different logger from us (logrus), will look into how we can better handle that to get consistent logging in a different issue (Was already an issue, nothing new due to this change).