feat: add support for PTR, MX and DS record types

[castorw]

What does it do ?

This PR allows use of the following record types:

• PTR - Reverse IP mapping to forward records,
• MX - Mail server record for domain,
• DS - Delegations signer (used to establish a chain of trust for DNSSEC)

Motivation

While setting up external-dns as provisioner for reverse zones I noticed that the controller gets in a conflict loop while trying to create records which already exist. It creates records and then in the next loop attempts to create these again. This is caused by a simple omission of allowed record types in the recordfilter.go file. This will allow users to manage wider range of records.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly
  • not much to update as the docs basically states that this should work

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[ivankatliarchuk]

I think you need to provide https://github.com/kubernetes-sigs/external-dns/blob/master/docs/contributing/bug-report.md.

The only record i'm not using is DS, rest are working fine. So not too clear what the problem is at the moment.

[castorw]

I am working with AWS provider, which does not load all record types due to the following call stack:

1. https://github.com/kubernetes-sigs/external-dns/blob/master/provider/aws/aws.go#L526
2. https://github.com/kubernetes-sigs/external-dns/blob/master/provider/aws/aws.go#L1450
3. https://github.com/kubernetes-sigs/external-dns/blob/master/provider/recordfilter.go#L21

Therefore using supported record types defined in provider package, which do not include PTR, MX or DS types. This causes the controller to write these records to Route53 and in the next run it attempts to write them again which causes errors as the records already exist. Caused by the fact that the records are filtered out during planning read from Route53.

[castorw]

Other option would be to allow these only for Route53 provider here. But not sure if there is a reason for these types not being allowed globally.

[castorw]

/assign @ivankatliarchuk

[castorw]

@ivankatliarchuk Do you think we can move forward with this?

[ivankatliarchuk]

I think you need to provide https://github.com/kubernetes-sigs/external-dns/blob/master/docs/contributing/bug-report.md.

The only record i'm not using is DS, rest are working fine. So not too clear what the problem is at the moment.

Still w8 to better understand the problem and risks.