Add REST API methods for alphabetical index

[osma]

Fixes #599

This adds two methods to the REST API:

/rest/v1/<vocab>/index/?lang=<language> -> list of letters available in the vocabulary alphabetical index, in the given language
/rest/v1/<vocab>/index/<letter>?lang=<language> -> list of terms/concepts for that letter in the vocabulary alphabetical index

Still TODO:

• implement limit and offset parameters
• update Swagger / OpenAPI spec
• address QA issues, if any

[codecov]

Codecov Report

Merging #976 into master will increase coverage by 0.11%.
The diff coverage is 74.19%.

@@             Coverage Diff              @@
##             master     #976      +/-   ##
============================================
+ Coverage     58.97%   59.08%   +0.11%     
- Complexity     1511     1517       +6     
============================================
  Files            32       32              
  Lines          4232     4263      +31     
============================================
+ Hits           2496     2519      +23     
- Misses         1736     1744       +8     

Impacted Files	Coverage Δ	Complexity Δ
controller/RestController.php	7.89% <74.19%> (+3.90%)	153.00 <6.00> (+6.00)

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
2 Code Smells

No Coverage information
0.0% Duplication

[joelit]

Looks good! I appreciate the thoroughness of the RestController tests.