Add movie external_ids

[SosthenG]

Add support for movie external_ids.
It was already implemented for tv, so it's just a copy/paste.

[codecov-io]

Codecov Report

Merging #232 (d5af888) into 4.0 (0335d97) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                4.0     #232      +/-   ##
============================================
+ Coverage     84.02%   84.07%   +0.04%     
- Complexity     2091     2096       +5     
============================================
  Files           194      194              
  Lines          5284     5300      +16     
============================================
+ Hits           4440     4456      +16     
  Misses          844      844              

Impacted Files	Coverage Δ	Complexity Δ
lib/Tmdb/Api/Movies.php	95.45% <100.00%> (+0.21%)	22.00 <1.00> (+1.00)
lib/Tmdb/Factory/MovieFactory.php	88.54% <100.00%> (+0.26%)	54.00 <0.00> (+1.00)
lib/Tmdb/Model/Movie.php	99.09% <100.00%> (+0.02%)	83.00 <2.00> (+2.00)
lib/Tmdb/Repository/MovieRepository.php	98.23% <100.00%> (+0.08%)	31.00 <1.00> (+1.00)

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[wtfzdotnet]

@SosthenG legend! Thanks for the pr :-)