Optional support for multi-value PTRs

[mochipon]

Multi-value PTR records is indeed supported by octoDNS.
The problem is if unsupported providers (e.g., PowerDNS, Google Cloud DNS) are used. In a backward compatible way, octoDNS will leave only one value.

https://github.com/octodns/octodns/blob/6890e3307c7246720820e4dc8f18edc2c8bcf164/octodns/provider/base.py#L91-L102

However, we cannot choose which to keep. The current implementation seems to keep the first value sorted by name, but this may not be what the user actually wants.

For instance, I might type v1001.rtr.example.com,rtr.example.com in the description field. In this case, rtr.example.com will always be selected, but is this the desired result for the operator?

[codecov]

Codecov Report

Merging #9 (ba4d2f7) into main (167f0a9) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main        #9   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           46        46           
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  Hits            46        46           

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[codeclimate]

Code Climate has analyzed commit ba4d2f7 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 100.0% (0.0% change).

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