chore: Add attoseconds accessor

[hassila]

Description

Add attoseconds accessor for instant, rename seconds accessor

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Minimal checklist:

• [x ] I have performed a self-review of my own code
• I have added DocC code-level documentation for any public interfaces exported by the package
• I have added unit and/or integration tests that prove my fix is effective or that my feature works

[shortcut-integration]

This pull request has been linked to Shortcut Story #858: Kill Timestamp from DataModel, use UTC Clock Instant from DateTime package.

[mr-swifter]

Will you add init from seconds and attoseconds in the same PR? or?

[github-actions]

Pull request benchmark comparison [ubuntu-latest] with 'main' run at 2022-11-28T16:59:33+00:00

[codecov]

Codecov Report

Merging #3 (f885861) into main (26f1193) will decrease coverage by 2.28%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
- Coverage   36.76%   34.48%   -2.28%     
==========================================
  Files           3        3              
  Lines         136      145       +9     
==========================================
  Hits           50       50              
- Misses         86       95       +9     

Impacted Files	Coverage Δ
Sources/DateTime/InternalUTCClock.swift	43.21% <0.00%> (-5.40%)	⬇️

Impacted Files	Coverage Δ
Sources/DateTime/InternalUTCClock.swift	43.21% <0.00%> (-5.40%)	⬇️

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[hassila]

Will you add init from seconds and attoseconds in the same PR? or?

f885861