[avro-schema] fix: enum logic (sanitize, remove collisions)

[joscha]

Currently, enums in Avro schemas are generated directly from the model, whether they are valid or not.

Unfortunately, Avro schema values follow tighter rules.

This pull request sanitizes enum values from the Swagger spec to follow the Avro schema by:

• replacing invalid characters with _
• prefixing values starting with numbers with _
• removing collisions that happen because of the above or otherwise by adding a numbered suffix.

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[joscha]

- to _

[joscha]

prefixed with _

[joscha]

count suffix

[wing328]

thanks for the PR.

cc @sgadouar

[joscha]

happy to express this in unit tests instead if you prefer @sgadouar