feat(pkger): add export support for tasks

[jsteenb2]

lo and behold, our last PR before an MVP can be realized. This is to supply tasks for exporting:

$ influx pkg export --tasks=04f90352681d3000
apiVersion: 0.1.0
kind: package
meta:
    description: ""
    pkgName: new_6868142840635702249
    pkgVersion: v1
spec:
    resources:
      - every: 1h
        kind: Task
        name: task1
        offset: 10m0s
        query: |-
            from(bucket: "rucket1")
             |> yield()

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)