Enable virtual path aliases for the asset server API

[rdw-software]

Goals:

• [x] A lookup table can be used to initialize the asset server API (map file data ID to GRF path)
• [x] If an entry exists, the mapped GRF path can be served when the file data ID (alias) is requested
• [x] If none exists, the requested URL is used directly (even if it results in a 404 error)

The purpose of this is twofold:

1. Avoid encoding issues (browsers use percent encoding, but I don't want to implement a percent decoder in Lua)
2. Can use arbitrary (human readable) file data IDs instead of Windows-encoded GRF path names

[rdw-software]

I think the easiest way is to just use the builtin routing functionality and define wildcard patterns, then let uws handle the rest.