Enable create cr ctl command to fetch certificate

[hzhou97]

What this PR does / why we need it:
This PR enables the user to specify --fetch-certificate and --output-cert-file flags to wait for the CertificateRequest that was created to be ready and store the certificate in a file.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

kubectl cert-manager: Added flags to wait for the CertificateRequest to be ready and store the certificate in a file.

/kind feature
/area ctl

[munnerz]

Once you in-line the code from PollUntilCRIsReadyOrTimeOut, I think you will be able to more easily access cr without having to call Get again (as we will have fetched it in that earlier 'wait' code).

We also shouldn't need to check CertificateRequestHasCondition either, as that will have already been verified above.

Once you remove these changes, the FetchCertificateFromCR function only consists of:

	// Store certificate to file
	err = ioutil.WriteFile(certFileName, req.Status.Certificate, 0600)
	if err != nil {
		return fmt.Errorf("error when writing certificate to file: %w", err)
	}

	fmt.Fprintf(ioStreams.Out, "Certificate has been stored in file %s\n", certFileName)

which I think is short and concise enough to be in-lined here, which should a) make this a bit simpler to read (less jumping between files 🎉) and b) mean we don't need to export new functions

[hzhou97]

I wrote this function with the separate fetch command in mind, thinking I could reuse this function.
I do agree that what you propose is cleaner if we are only using it in this context, but I'm not sure if we want to duplicate the writing to file logic in two places.

[munnerz]

This wasn't added here but.. yes, definitely 😄 especially as this output is user-facing in their CLI, so it's good to have the actual text encoded here so we know when we change it (either by accident or intentionally). It also helps us avoid doing something like attempting to print a structure or a pointer or something to the CLI, which is really confusing for people 😅

[james-w]

Thanks for the PR, I think it looks good, I just have some fairly minor suggestions.

[james-w]

Do we no longer want this restriction?

[hzhou97]

We don't, the team mentioned people might prefer other file extensions, so it would be easier to just not check at all.

[munnerz]

Approval = this should exist in our codebase, which it should, so adding an approved label and we can get lgtm on there soon 😄

/approve

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hzhou97, munnerz

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[james-w]

/lgtm

Thanks!

[hzhou97]

/kind feature