Configuration: add ability to customize github release body

[Marco Ieni]

Motivations

Now release-plz populates the github release body with the changelog.
People might want different things.

Solution

We could have a configuration field called git_release_body. It is a tera template where people can customize how their releases look. Similar to git_tag_name.
A variable called {{ changelog }} should be populated with the changelog entry of the release.
The default configuration is git_release_body = "{{ changelog }}", which is the current behavior.

An example configuration might be

git_release_body = """Hey, welcome to the new release, here's what changed:
{{ changelog }}
"""

[giangndm]

@MarcoIeni Can we split the changelog into smaller sections such as features, bugs, chores, or contributors? It would be more flexible that way.

[Marco Ieni]

Maybe we should provide the same level of customizability we provide for the changelog body field. For now, let's just provide the changelog variable, because I feel like it could be useful anyway. And that's what most of the people want to use.
Then we can reason about more customizability. What do you think?

[Marco Ieni]

Closed by 2733209