feat: add option to use only new format TXT records

[malpou]

Description

Adds a new flag --txt-new-format-only that allows configuring external-dns to only create TXT records in the new format, containing record type information. This helps reduce the number of DNS records created when using external-dns at scale, addressing provider-specific record limits.

By default, the TXT registry creates both old and new format records for backwards compatibility. The new format includes the record type in the name (e.g., "a-" prefix for A records). With this change, users can opt-in to using only the new format when all their external-dns instances support it.

Key changes:

• Add --txt-new-format-only flag (defaults to false for backwards compatibility)
• Add configuration option to TXTRegistry
• Update TXT record generation to respect the new setting
• Add tests for the feature
• Maintain existing behavior by default

Example usage:

# Default behavior - creates both old and new format records
external-dns

# Enable new format only - reduces number of TXT records
external-dns --txt-new-format-only

Note: AAAA records already only create new format records regardless of this setting, as they have special handling.

Fixes #3167
Fixes #4636

Checklist

• Unit tests updated
• End user documentation updated

[linux-foundation-easycla]

• ✅login: malpou / (0d56712)
• ✅login: malpou / (0d56712, c7154c9)

The committers listed above are authorized under a signed CLA.

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