Cosmos/FullTextSearch: add model builder option for default language

[maumar]

Per documentation (https://learn.microsoft.com/en-us/azure/cosmos-db/gen-ai/full-text-search#full-text-policy) this is one of the container settings. For now only en-US is supported and to make this configurable we'd need something like ContainerBuilder (and build action which uses it).

[roji]

@maumar do you see us implementing this later in 10, i.e. not part of the initial feature?

[maumar]

It's fairly useless at the moment, since en-us is the only supported language, and adding superfluous container option might be confusing to some users, so I'm not sure. The work itself is very small and simple.