Classify PRs using .projects field

[palas]

Currently the cardano-api repo has 4 projects in it, and the generate-pr-changelogs.sh script collects them mixed, this makes it hard to create independent changelogs for each of the projects.

This PR takes advantage of the projects field if available, and it classifies the change log entries in separate lists for each of the projects in the entries listed.

• If a project doesn't have a project specified it will be classified under Project not specified
• If a project has several projects specified it will be classified under each of them

See this related PR: IntersectMBO/cardano-api#950

I recommend looking at this PR by disabling whitespace changes.

[Jimbo4350]

Can you first confirm with a previous release that this generates the same change logs?

[palas]

Can you first confirm with a previous release that this generates the same change logs?

I've checked and it seems to work. I tried it for 10.16.0.0, and it produced the same exact entries that are in the changelog.md for that release. With the difference now it writes them under ## Project not specified.