Allow configuring TXT registry to only generate the "new" format of TXT records

[mjlshen]

What would you like to be added:
Currently when using the TXT registry, two TXT records (old + new format) are being created for every record the external-dns creates:

external-dns/registry/txt.go

Lines 212 to 242 in c45d0de

	// generateTXTRecord generates both "old" and "new" TXT records.
	// Once we decide to drop old format we need to drop toTXTName() and rename toNewTXTName
	func (im *TXTRegistry) generateTXTRecord(r *endpoint.Endpoint) []*endpoint.Endpoint {
	endpoints := make([]*endpoint.Endpoint, 0)
	if !im.txtEncryptEnabled && !im.mapper.recordTypeInAffix() && r.RecordType != endpoint.RecordTypeAAAA {
	// old TXT record format
	txt := endpoint.NewEndpoint(im.mapper.toTXTName(r.DNSName), endpoint.RecordTypeTXT, r.Labels.Serialize(true, im.txtEncryptEnabled, im.txtEncryptAESKey))
	if txt != nil {
	txt.WithSetIdentifier(r.SetIdentifier)
	txt.Labels[endpoint.OwnedRecordLabelKey] = r.DNSName
	txt.ProviderSpecific = r.ProviderSpecific
	endpoints = append(endpoints, txt)
	}
	}
	// new TXT record format (containing record type)
	recordType := r.RecordType
	// AWS Alias records are encoded as type "cname"
	if isAlias, found := r.GetProviderSpecificProperty("alias"); found && isAlias == "true" && recordType == endpoint.RecordTypeA {
	recordType = endpoint.RecordTypeCNAME
	}
	txtNew := endpoint.NewEndpoint(im.mapper.toNewTXTName(r.DNSName, recordType), endpoint.RecordTypeTXT, r.Labels.Serialize(true, im.txtEncryptEnabled, im.txtEncryptAESKey))
	if txtNew != nil {
	txtNew.WithSetIdentifier(r.SetIdentifier)
	txtNew.Labels[endpoint.OwnedRecordLabelKey] = r.DNSName
	txtNew.ProviderSpecific = r.ProviderSpecific
	endpoints = append(endpoints, txtNew)
	}
	return endpoints
	}

I would like to propose a flag (or some other way) to configure the TXT registry to only use the new format.

Why is this needed:
I am trying to use external-dns to manage DNS records at scale and am running into cloud provider limits, e.g. by default Azure public DNS Zones allow for a maximum of 10,000 record sets. This quota can be increased, but if external-dns can be configured so that the TXT registry only creates one additional record, this would be a massive improvement for my use-case.

[mloiseleur]

@mjlshen Maybe you will be interested in #4575.
Using CR as a registry could help overcome this kind of limitations, too.

[maxwell-gregory]

Having 3 records to sync with external-dns and my DNS provider is causing rate limiting. Any hopes on a fix for getting rid of this old txt record style?

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