implement functions for sources endpoint

[sboysel]

Addresses #24

[codecov-io]

Codecov Report

Merging #34 into master will decrease coverage by 0.15%.
The diff coverage is 98.18%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   99.81%   99.65%   -0.16%     
==========================================
  Files          21       24       +3     
  Lines         538      582      +44     
==========================================
+ Hits          537      580      +43     
- Misses          1        2       +1

Impacted Files	Coverage Δ
R/fredr_sources.R	100% <100%> (ø)
R/fredr_source_releases.R	100% <100%> (ø)
R/fredr_source.R	100% <100%> (ø)
R/validation.R	98.51% <94.73%> (-0.7%)	⬇️

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