feat: Support concatenation for root keys of the default message and loaded config

[speeddragon]

Allow adding an item to the already defined default config in the hb_opts module. For example, adding another route or store to the already configured one.

• Added root-only +key append and key+ prepend behavior in src/core/resolver/hb_opts.erl:677.
• Binary, atom, and map values are wrapped in lists; lists remain unchanged.
• +key+ and nested decorated keys are untouched.
• Decorated root keys are removed from the merged configuration.
• Integrated the merge into HB_CONFIG startup loading in src/core/http/hb_http_server.erl:39.

[speeddragon]

I was thinking about the use case for "stores", which is a structure with more than 1 key, and we accept either a map or a list.

For the on use case, we access it via the map, and we want to add a new entry (e.g., cache-write) to the on hook configuration. For this, we need another pattern.

Suggestion

++ - Concatenate to list.
+ - Add entry on map.

[samcamwilliams]

+ on a message should be irrelevant because the default is to merge not replace anyway?

[speeddragon]

+ on a message should be irrelevant because the default is to merge not replace anyway?

The default on edge is to replace.
The default on this branch is to concatenate into a list.

on hook is expecting it to be a map, so if we want to merge with the current default, we need to provide a different behaviour. Checking the previous type (list, map, ...) doesn't work, because <<"stores">> supports both (map and list).