feat: Added go package to support v2 ingestion

[eddie-knight]

Part of ossf/security-insights-spec#97

This adds support for the v2 release of Security Insights spec. It will read the specified security insights file, look for a value in header.project-si-source, and (if found) override the target SI with the project information from the SI source file.

Example usage:

package main

import (
	"fmt"

	"github.com/ossf/si-tooling/v2/si"
)

func main() {
	out, err := si.Read("ossf", "security-insights-spec", ".github/security-insights.yml")
	if err != nil {
		fmt.Print(err)
		return
	}
	fmt.Print(out)
}