feat: export generated builds as json

[chreck]

I encountered a problem when I tried to compile a list of all initiated builds for a GitHub release note. Simply using the build slug to summarize all the information was proving to be difficult. Therefore, I decided to extract additional data on all initiated builds and export it in a JSON format. As JSON format it makes the reuse more flexible for future Bitrise workflow steps.

Checklist

• I've read and followed the Contribution Guidelines
• step.yml and README.md is updated with the changes (if needed)

Version

Requires a MAJOR/MINOR/PATCH version update

Context

Exporting only the build slug is not enough for future Bitrise workflow steps.

Resolves: #39

Changes

• use package encoding/json
• save the builds in an array
• marshal to a JSON string
• add new export definition in step.yml

Investigation details

No investigation details

Decisions

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