Support custom OpenAPI "example" keyword in AJV validation

[LoicPoullain]

Issue

Resolves #1192.

Solution and steps

• [x] Add the support of the keyword in AJV validation instance

Checklist

• [x] Add/update/check docs (code comments and docs/ folder).
• [x] Add/update/check tests.
• [x] Update/check the cli generators.

[codecov-commenter]

Codecov Report

Base: 98.86% // Head: 98.86% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (fe01a09) compared to base (a6f15e5). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v3-1-0 #1194 +/- ## ======================================= Coverage 98.86% 98.86% ======================================= Files 96 96 Lines 1668 1669 +1 Branches 388 388 ======================================= + Hits 1649 1650 +1 Misses 19 19 ``` | [Impacted Files](https://codecov.io/gh/FoalTS/foal/pull/1194?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=FoalTS) | Coverage Δ | | |---|---|---| | [...ges/core/src/common/validation/get-ajv-instance.ts](https://codecov.io/gh/FoalTS/foal/pull/1194/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=FoalTS#diff-cGFja2FnZXMvY29yZS9zcmMvY29tbW9uL3ZhbGlkYXRpb24vZ2V0LWFqdi1pbnN0YW5jZS50cw==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=FoalTS). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=FoalTS)

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