Allow the usage of git-trailers for changelog messages

[poljar]

This PR modifies the way git-cliff parses commits and produces changelog entries, please take a look at the updated CONTRIBUTING.md file for more info.

In short, it allows us to write a commit like this:

fix: Use a constant-time Base64 encoder for secret key material

This patch fixes a security issue around a side-channel vulnerability[1]
when decoding secret key material using Base64.

In some circumstances an attacker can obtain information about secret
secret key material via a controlled-channel and side-channel attack.

This patch avoids the side-channel by switching to the base64ct crate
for the encoding, and more importantly, the decoding of secret key
material.

Security-Impact: Low
CVE: CVE-2024-40640
GitHub-Advisory: GHSA-j8cm-g7r6-hfpq

Changelog: Use a constant-time Base64 encoder for secret key material
to mitigate side-channel attacks leaking secret key material.

To produce a changelog entry like this:

### Security

- Use a constant-time Base64 encoder for secret key material to mitigate
  side-channel attacks leaking secret key material.
  (Low, [CVE-2024-40640](https://www.cve.org/CVERecord?id=CVE-2024-40640), [GHSA-j8cm-g7r6-hfpq](https://github.com/matrix-org/vodozemac/security/advisories/GHSA-j8cm-g7r6-hfpq))

[Hywan]

Looks great.

[bnjbvr]

looks good!

[poljar]

@dkasak do you want to take a look at this or should I merge?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.55%. Comparing base (72f4235) to head (baf9cc9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #165   +/-   ##
=======================================
  Coverage   90.55%   90.55%           
=======================================
  Files          34       34           
  Lines        1906     1906           
=======================================
  Hits         1726     1726           
  Misses        180      180           

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[poljar]

[bnjbvr]

🚢 🐑 🥳

[dkasak]

Looks good. Left some suggested rephrasings.