added rumor timestamp to serializers

[DanielTavaresA]

Following the addition of timestamps to rumor messages, serialization of this field has been added

[github-actions[bot]]

Pull reviewers stats

Stats of the last 30 days for popstellar:		User	Total reviews	Time to review	Total comments
	K1li4nL 🥇	17 ▀▀▀▀	1d 19h 11m)) ▀▀	12 ▀▀▀
	arnauds5 🥈	5 ▀	7h))	4 ▀
	emonnin-epfl 🥉	5 ▀	8h 20m))	9 ▀▀
	onsriahi14	4 ▀	1d 1h 53m)) ▀	4 ▀
	MariemBaccari	2 ▀	22h 39m)) ▀	6 ▀
	simone-kalbermatter	2 ▀	23h 35m)) ▀	0
	matteosz	2 ▀	1d 5h 29m)) ▀	3 ▀
	sgueissa	1	49m))	3 ▀
	DanielTavaresA	1	2d 1h 22m)) ▀▀	0

^{⚡️ Pull request stats}

[github-actions[bot]]

🟠 There seems to be some formatting issues that must be applied from PR https://github.com/dedis/popstellar/pull/1940 first.

[sonarcloud[bot]]

Quality Gate passed for 'PoP - PoPCHA-Web-Client'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarcloud[bot]]

Quality Gate passed for 'PoP - Be2-Scala'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
88.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarcloud[bot]]

Quality Gate passed for 'PoP - Be1-Go'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarcloud[bot]]

Quality Gate passed for 'PoP - Fe2-Android'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud