trailers: make separators configurable

[glehmann]

fix #6837

Checklist

If applicable:

• [x] I have updated CHANGELOG.md
• [x] I have updated the documentation (README.md, docs/, demos/)
• [x] I have updated the config schema (cli/src/config-schema.json)
• [x] I have added/updated tests to cover my changes

[dolangish]

I'm a C++ dev and not a rust guy, but to my eyes this looks like it addresses my FR #6837 and is in line with how the git documentation describes their trailer separators. Thanks for the speedy turnaround on this!