Add relnotes in JSON to draft pull request

[puerco]

What type of PR is this?

/kind feature

What this PR does / why we need it:

krel release-notes will now push the JSON version of the release notes draft to k/sig-release when generating the release notes draft pull request.

Which issue(s) this PR fixes:

Ref: #1087
Ref: kubernetes/sig-release#1373 (comment)

Special notes for your reviewer:

/hold until 1.20 is cut

Two details with this change:

1. This PR will write the full version of the notes in JSON to be read and copied to the bucket by the change in Fetch remote JSON release notes for official releases #1815. But I think we need to generate the JSON file for the website, containing only the current tag's notes (like we used to do with these files here). I assume we will read these from each archived release. CCing Chris to keep everyone in the loop.
2. The JSON file is not prettified. I think this is the right time to get rid of the npm prettifier dependency and switch to an all go solution. Let's discuss how we want/need the files to look like.

/cc @saschagrunert @wilsonehusin @cartyc

Does this PR introduce a user-facing change?

- The Release Notes draft pull request now includes the JSON version of the release notes draft which 

[saschagrunert]

Code LGTM, but let me be your linter:

cmd/krel/cmd/release_notes.go:334:1: cyclomatic complexity 31 of func `createDraftPR` is high (> 30) (gocyclo)
func createDraftPR(repoPath, tag string) (err error) {
^

[puerco]

I've split the commit creation to its own function to lower the complexity.

I'm thinking on doing a major refactor to krel release-notes to implement the client/implementation pattern now that we are doing some work on it again.

[saschagrunert]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: puerco, saschagrunert

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• cmd/krel/OWNERS [puerco,saschagrunert]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[puerco]

/hold cancel