feat(forms): Add SLA field configuration for form destination

[ccailly]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets

Possible configurations :

"Specific SLA" display a secondary select to let the user pick the chosen SLA :

[AdrienClairembault]

I didn't review it yet, just one small question: is it normal that the cypress tests were fixed by removing the .only directive ?

Ideally, tests should be able to run independently (see https://docs.cypress.io/guides/references/best-practices#Having-Tests-Rely-On-The-State-Of-Previous-Tests).