feat(source): add unstructured source

[ivankatliarchuk]

What does it do ?

A new unstructured source that creates DNS records from any Kubernetes custom resource using Go templates.

Supporting every possible ingress or product isn’t practical. For example, it’s fairly common to have custom products, and ExternalDNS doesn’t really fit that model today, as it doesn’t provide even basic support for custom sources.

Added flags

• --unstructured-resource=
• --fqdn-target-template=
• --fqdn-host-target-template=

Motivation

We shouldn't have to wait for external-dns to add built-in support for some specific CRDs. With this source, they can point external-dns at any resource (KubeVirt VMs, Crossplane resources, MetalLB pools, etc.) and use templates to extract hostnames and targets from .spec or .status fields.

Fixes #3437
Fixes #5104

First attempt #3490. That was abandoned

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

I could slice this PR, move tests to another PR and 400+ lines in docs

┌──────────┬───────┐
│ Category │ Lines │
├──────────┼───────┤
│ Non-test │ +515  │
├──────────┼───────┤
│ Test     │ +1728 │
├──────────┼───────┤
│ Ratio    │ 1:3.3│
└──────────┴───────┘

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[coveralls]

Pull Request Test Coverage Report for Build 21788334937

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 119 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.1%) to 79.053%

---

Files with Coverage Reduction	New Missed Lines	%
openshift_route.go	1	83.97%
apis/externaldns/types.go	1	99.62%
apis/externaldns/validation/validation.go	2	97.62%
fqdn/fqdn.go	5	93.59%
testutils/endpoint.go	16	71.29%
store.go	94	46.28%

Totals
Change from base Build 21559764040:	-0.1%
Covered Lines:	16243
Relevant Lines:	20547

---

💛 - Coveralls