[expression-language] Improve map access with dot

[gcusnieux]

🌈 Feature

As a 'API Publisher'

I want to 'use simple syntax to access map data with expression language expression'

So that '...'

🌄 Additional information

It would be simpler to access to map value with dot syntax like {#context.attributes.api}
Default syntax {#context.attributes['api']} should be always work

🚀

[mouligno]

LGTM