Add a way to set default rate limit per path prefix

[stevehu]

So that users won't need to set up for every path prefix. We also need to allow the user to overwrite a particular path prefix.

[stevehu]

After reviewing the server-limit implementation, I realized that it is not possible to configure default limits on a per–path-prefix basis. In a light-gateway instance that manages multiple APIs and applications, path prefixes are derived from API URLs and are used to categorize requests into different buckets. These rules are highly flexible.

For example, we may define different path prefixes with different limits when v1 is implemented using REST and v2 is implemented using GraphQL:

/petstore/v1
/petstore/v2

Alternatively, we may choose to apply a single limit across all versions of the Petstore API:

/petstore

As a result, unless all required path prefixes are explicitly listed in the server configuration, it is not possible to define a default limit per path prefix. If all path prefixes must be listed anyway, then there is no additional effort required to configure limits per path prefix directly using the same structure.