Supports for user specific bitmasks for "map-like-directives"

[dengsh12]

Following the discussion here:
https://github.com/nginxinc/nginx-go-crossplane/pull/117/files#r1700552067

Is your feature request related to a problem? Please describe

Users may have a module not in crossplane, say md. md has a "map-like-directive" mld. It acts in this way:

mld{
a 1
b 2
c 3
}

currently crossplane can't parse it.

Describe the solution you'd like

Provide an interface for users to specify rules of "map-like-directive".