Support commit list substitution in pull request description templates

From BCLOUD-11571:

Prior to activating the templates, I could use a commit message like the following:

Derezz the master control program

MCP turned out to be evil and had become intent on world domination.
This commit throws Tron's disc into MCP (causing its deresolution)
and turns it back into a chess game.

And it would use the first line (subject line) as the PR title and the message would automatically be inserted into the description. If I've already written a detailed commit message, I don't want to have to re-type it for the PR.

Latest update from Atlassian 2023-07-20

We have added a capability that lets you substitute a list of commit messages by using the variable {{commit_messages}} in a default pull request description template. Docs are here: https://support.atlassian.com/bitbucket-cloud/docs/pull-request-and-merge-settings/