Add description on enum constants

[clementdenis]

This PR allows to set a description on enum values in API method parameters, as allowed in the discovery format.

• The DiscoveryGenerator already supported enum descriptions.
• The OpenAPI format does not support descriptions on enum values (yet).

PS: we tried to implement description for enums in Swagger here, as described in Swagger documentation for enums, using the description field, but we preferred not to include it yet.

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this State. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[kaldebert]

I'm ok with my commits being contributed to this project.

[tangiel]

Looks like merging the other PR puts this in conflict. Please fix.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@be24045). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #133   +/-   ##
=========================================
  Coverage          ?   79.86%           
  Complexity        ?     1662           
=========================================
  Files             ?      156           
  Lines             ?     5568           
  Branches          ?      716           
=========================================
  Hits              ?     4447           
  Misses            ?      843           
  Partials          ?      278

Impacted Files	Coverage Δ	Complexity Δ
.../api/server/spi/config/model/SchemaRepository.java	92.15% <100%> (ø)	28 <0> (?)
...e/api/server/spi/discovery/DiscoveryGenerator.java	87.74% <100%> (ø)	52 <0> (?)

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[kaldebert]

@tangiel The merge includes the fix for the currently failing master.
The last commit covers the missing case for description of $ref fields.

[tangiel]

Is there a reason for changing this indent?

[tangiel]

Put 2018 here.