Add missing card properties

[TonioOoOo]

Changes summary

• Add optional level to Header Block ;
• Add SlackIcon and sub text to cards block ;
• Add Container block ;
• Add Callout block

Note The callout isn't documented yet, but it's already available in the builder (here),

[stoiveyp]

Morning @TonioOoOo - great catch, would you mind updating or adding tests so these fields are covered moving forward?

[TonioOoOo]

I've set it up for testing. I took the opportunity to add the Container (ReleaseNote) and a Callout.

Note The callout isn't documented yet, but it's already available in the builder (here), which allowed me to reverse-engineer its properties. I tested it, and it works. It will need to be double-checked once the official documentation is released.

[TonioOoOo]

Adding header level missing property.