chloggen: allow specifiying PR number in the template

[joaopgrassi]

Today, the template has a field for specifying an issue https://github.com/open-telemetry/opentelemetry-go-build-tools/blob/main/chloggen/internal/chlog/entry.go#L41. In many repos using chloggen, the same field is also used for PR numbers (when there's no issue for example).

In the semconv repo, we want to generate links to the issue/PR numbers in the changelog to make it easier to navigate. Because the field issues today can hold both, I think it is not possible to generate the changelog the way we want. For example:

{{- define "entry" -}}
- `{{ .Component }}`: {{ .Note }} (
{{- range $i, $issue := .Issues }}
{{- if $i }}, {{ end -}}
[#{{ $issue }}](https://github.com/open-telemetry/semantic-conventions/issues/{{ $issue }})
{{- end -}}
)

So, it would be great if we could have another field pull_request in the template, so we could use it instead of the issue for semconv (as there it's almost always more interesting to see the actual changes in the changelog).

For context: open-telemetry/semantic-conventions#1627

[joaopgrassi]

Alright, I was unaware that GitHub automatically redirects correctly PRs/Issues, so #633 redirects to this issue, same as as #634 redirects to the PR.

I guess then this is not needed as we can just use the issues field.