Migrate JSON Merging to its own package

[jamietanna]

• Reduce unnecessary dependencies on github.com/apapsch/go-jsonmerge/v2
• Requires upstream changes in oapi-codegen

[come-maiz]

The repository github.com/apapsch/go-jsonmerge doesn't have the issues enabled, the author didn't send the changes upstream, and it hasn't been changed in 4 years. Adopting the code in this repository feels safer.

[lebaptiste]

@jamietanna Do we have any update?

[jamietanna]

No, if the issue hasn't been updated, there's no updates :) Is there any reason you need this? Ie seeing increased dependency bloat, CVEs etc?