updatecli: link to the original Pull Request

[v1v]

What

Add the original Pull Request that contains the changes in the specs to be in the description of the Pull Request created with the updatecli automation. This will help with track what APM Agents use it.

Test

Given the updatecli and gh cli

When running

$ GIT_USER=foo \
GIT_EMAIL=1 \
GITHUB_TOKEN=*** \
updatecli apply --config .ci/updatecli.d/update-specs.yml --push=false --debug

Then

...
✔ content: found from file "https://github.com/elastic/apm-data/commit/main.patch":
From 5d596b8bb0cbf31aac8ea65e611a8d5d7d5b482d 
...
----
https://github.com/elastic/apm-data/pull/178
----
...