⏰ Add td_*seconds serde Serialize and Deserialize functions for TimeDelta

[tosti007]

Fixes #117

This PR copy-pastes the current implementation for DateTime such that we have equal implementations for DeltaTime.

I opted to add the logic to the existing serde.rs file. However, I think this might be the wrong place and a better solution would be to move the main.rs's serde module combined with this serde.rs file to a separate src/serde.rs file, thoughts?

PS:
I am just doing a drive-by PR on this crate, so I don't really know what the exact requirements are regarding testing ect. Please let me know!

[djc]

I'd like to see at least one basic round-tripping test for each of these.

[pistomat]

I can't wait for this PR to be merged!

[tosti007]

I can't wait for this PR to be merged!

Thanks for the ping, I totally forgot about this. I'll see if I can get it in a good state somewhere this week.

[codecov]

Codecov Report

Attention: Patch coverage is 78.55711% with 107 lines in your changes missing coverage. Please review.

Project coverage is 90.26%. Comparing base (3b4af71) to head (51ee664).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/datetime/serde.rs	80.00%	98 Missing ⚠️
src/lib.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1652      +/-   ##
==========================================
- Coverage   90.59%   90.26%   -0.33%     
==========================================
  Files          38       38              
  Lines       16028    16463     +435     
==========================================
+ Hits        14520    14860     +340     
- Misses       1508     1603      +95     

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[tosti007]

I see codecov says no, I haven't really used that before. Does that mean it just requires more tests which touch the added codepaths?