Enable `spin add` for Redis templates about spin HOT 3 CLOSED

(Incidentally this would have been a challenge for HTTP too had wise Lann not wisely made base optional, the wise fellow.)

from spin.

The documentation in the https://developer.fermyon.com/spin/v2/triggers file will need to be amended if/when this change is implemented. The triggers page currently gives the following instructions re: multiple trigger applications via spin new and spin add:

Currently, only the HTTP template supports the spin add command. If you wish to use spin add to build an application with multiple triggers, including a Redis trigger, you should begin with the Redis trigger application. Then, spin add your HTTP trigger component(s) as demonstrated below. Please note that additional Redis trigger components cannot be added via spin add at this time.

# Start with a Redis trigger application
$ spin new -t redis-rust trigger-example
Description: A multiple trigger example
Redis address: redis://localhost:6379
Redis channel: one
# Change into to the application directory
$ cd trigger-example 
# Create HTTP trigger application
$ spin add -t http-rust http-trigger-example  
Description: A HTTP trigger example
HTTP path: /...

from spin.

Looks like there's a smol challenge here.

A Redis application requires an application.trigger.redis.address entry. When we create a new spin.toml file (spin new), we can safely prompt for that and include that and all is well.

However, when we add a component to an existing spin.toml file, we need to be smarter. We should prompt for and add the address only if that entry doesn't yet exist.

The subsystems to do this don't currently exist in the templates crate. (Which is not to say they can't be added - they can definitely be added, it just means there's more to this than making a component snippet.)

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