Support for more SMS validity period formats

[ah9142]

Hello.

If you are interested in accepting pull requests,
I implemented encode/decode support for the absolute SMS validity period format
as well as support for 2/3 enhanced SMS validity period formats.

I think the main thing you might want to check is the public Message struct.
New parameter fields AbsoluteVP and EnhancedVP for the new formats were added.
This way of adding them is simple and preserves backwards compatibility.
However, if you have any objections, then feel free to propose another way and let's see how to go forwards.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 58.69565% with 38 lines in your changes missing coverage. Please review.

Project coverage is 34.76%. Comparing base (379970a) to head (8af6aa5).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
sms/validity_period.go	59.18%	16 Missing and 4 partials ⚠️
sms/sms.go	56.52%	8 Missing and 2 partials ⚠️
sms/sms_submit.go	63.15%	6 Missing and 1 partial ⚠️
at.go	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   36.13%   34.76%   -1.38%     
==========================================
  Files          18       21       +3     
  Lines        1605     1795     +190     
==========================================
+ Hits          580      624      +44     
- Misses        940     1080     +140     
- Partials       85       91       +6     

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[ah9142]

@xlab a gentle reminder if you might take a look at this at some point.

[xlab]

@ah9142 apologies for the delay, I am actually happy to accept pull requests and new features there. Having more validity period options is a great feat, and thanks for keeping compatibility.

I don't have unfortunately hardware means to validate this, but if it works for you that's cool!

[ah9142]

No problem. I haven't been working on anything at related since, but good to have this merged to trunk though.