fix: convert url variables to colon x

[bzp2010]

Description

Currently, the path variable in OpenAPI will be converted to *, its implementation on APISIX, which allows for arbitrary strings containing slash, which potentially doesn't fit our needs.

So this PR actually replaces variables like /order/{order_id} with /order/:order_id, which is more appropriate.

Specifically:

The original implementation:

/order/{order_id} => /order/*
/order/{order_id}/good/{good_no} => /order/*/good/*

While this does work, it will actually allow requests like this to be matched:

/order/12345/blahblah => /order/*
/order/12345/54321/good/12345 => /order/*/good/*

I.e., allowing * paths like xxx/xxx to be matched, which could change the semantics of the URL and pose a security risk.

Variables such as :xxx are not allowed to contain /, i.e., they match at most one level of the directory on the URL.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible