Upload all pipelines present in the input

[DrJosh9000]

Description

Allow pipeline YAML containing more than one document. When using buildkite-agent pipeline upload with such an input, upload all of them.

Context

https://linear.app/buildkite/issue/PS-795

Changes

• Update go-pipeline to v0.14.0
• Change parseAndInterpolate to use pipeline.ParseAll and return an iterator
• Change the action to put most of the work in a loop
• Move dry-run output setup outside of the loop so that the output YAML will consist of multiple correctly-separated documents

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[wolfeidau]

Nice work, looks really clean with the iterator 👍🏻 🚀