Ability to use existing/reference() inside lambda functions

[slavizh]

Is your feature request related to a problem? Please describe.
Although now lambda functions have support for index they are missing the ability to use existing syntax or reference() function within them. That is a gap that I think should be closed. There are a lot of limitations with for() syntax and lambda functions are able to overcome most of them with the exception of using existing/reference. It will make bicep a lot easier to use if these limitations are just removed.

Describe the solution you'd like
Ability to use existing syntax reference and reference() function inside lambda functions.

[stephaniezyen]

Dup of #14106