configuration for native queries

[hallettj]

Adds configuration for native queries, and includes native queries in schema response. Does not implement native queries in the query handler.

This changes the way the configuration directory is read:

• schema.{json,yaml,yml} is parsed according to configuration::Schema
• all json and yaml files in the native_queries subdirectory are parsed according to configuration::native_queries::NativeQuery, and are combined into a Vec
• the configuration::read_directory function combines the above results into one configuration::Configuration value

According to Rikin's feedback I changed the name of the configuration::Metadata type to configuration::Schema.

https://hasurahq.atlassian.net/browse/MDB-21

[dmoverton]

Looks good