Enhancement: Implement ChainUriRetriever

[localheinz]

This PR

• implements a ChainUriRetriever

Related to ergebnis/composer-normalize#80.

[codecov]

Codecov Report

Merging #102 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master   #102   +/-   ##
=======================================
  Coverage       100%   100%           
- Complexity      127    135    +8     
=======================================
  Files            28     30    +2     
  Lines           392    410   +18     
=======================================
+ Hits            392    410   +18

Impacted Files	Coverage Δ	Complexity Δ
src/Exception/UriRetrieverRequiredException.php	100% <100%> (ø)	1 <1> (?)
...rc/JsonSchema/Uri/Retrievers/ChainUriRetriever.php	100% <100%> (ø)	7 <7> (?)

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