luci-lua-runtime: proxy all dispatcher context property accesses

The ucode side dispatcher environment might not be fully populated yet
when the emulated Lua dispatcher is loaded, leading to `context.requested`
and some other properties to be `nil`.

Expose all properties through metatable `__index` lookups instead to
ensure that the Lua side dispatcher always sees the latest values.

Fixes: #6100
Signed-off-by: Jo-Philipp Wich <jo@mein.io>