switch configuration types to maps

[hallettj]

In some earlier feedback @dmoverton pointed out it would be better to store native queries in maps, as opposed to vecs. I thought the same logic would apply to some of the other configuration fields. It's also been bugging me that the configuration formats didn't match pattern in ddn and ndc-postgres in that we were using lists of named objects instead of maps.

I switched over some types from Vec<StructWithNameField> to BTreeMap<String, StructWithoutNameField>. To smooth things out I also added a WithName type that can wrap any struct, and adds a name field that appears at the same level as the underlying struct's fields in serialized formats. That lets us deserialize either without a name (for example if we're getting names from map keys in schema.json instead of from map value fields), or with a name (like if we split up schema configuration to put info for each collection into it's own file, we'll want to read the name from a field in the file.) I used that method to update read_subdir_configs in directory.rs so that it automatically grabs a name from each file.

The changes ended up being more extensive than I'd hoped so let me know if it's too much. But since this changes configuration formats I thought if we want to make this change it's better not to wait.

https://hasurahq.atlassian.net/browse/MDB-91

[dmoverton]

Looks good to me