fix(pullrequests) validate that specified targets exist in the manifest

[dduportal]

Fix #442

Test

To test this pull request, create the following manifest:

title: Bump Golang Version

scms:
  default:
    kind: github
    spec:
      user: updatecli
      email: me@olblak.com
      owner: updatecli
      repository: updatecli
      token: 'SuperSecretToken'
      branch: main

sources:
  latestVersion:
    name: Get Latest Version
    kind: shell
    spec:
      command: echo 1.0.0

targets:
  release-sandbox:
    name: 'Update last version'
    sourceID: latestVersion
    kind: shell
    spec:
      command: echo
    scmID: default

pullrequests:
  default:
    title: 'PR Title Here'
    kind: github
    scmID: default
    targets:
      - foo

you can run the following commands:

go build -o ./dist/updatecli && ./dist/updatecli diff --config manifest.yaml

Additional Information

Tradeoff

Potential improvement