feat: add support to use issue template from registry index for pack yank

[burnerlee]

Summary

This PR adds support to read the issue template from registry-index while running pack buildpack yank ....

In case the template doesn't exist it falls back to the harcoded template. The hardcoded template has been updated to include yank=true when .yank is true.

Output

Before

Issue Body:

id = "foo-ns/foo-bp"
version = "1.2.3"

After

Issue Body:

```
id = "foo-ns/foo-bp"
version = "1.2.3"
yank = true
```

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

resolves #2461

[burnerlee]

@dmikusa @jjbustamante could you PTAL?

[jjbustamante]

I would at least throw a warning message in the log to inform that there was some issue parsing the template, and we are falling back to a hardcoded template

[jjbustamante]

Hi @burnerlee!

Thanks for this PR, it looks good to me. Could you add some test coverage? especially for unhappy paths, I would like the users to easily provide logs or something if they have issues with the templates.

[jjbustamante]

Also do not forget to signoff your commits for the DCO check to pass

[burnerlee]

Sure let me add some unit tests