Feature - Support Decode for env var init

[sagiegurari]

Features Description
To enable defining env vars based on value mapping of other env vars, need to introduce a decode env capability.

Describe the solution you'd like
We could define the source value which can be an expression holding env vars and text, a value to value mappings and a default value.

Code Sample

[env]
PLATFORM_EXTERNAL_EXT_SUFFIX={decode={source = "${CARGO_MAKE_RUST_TARGET_OS}", default = "", mapping = {"linux" = "so", "macos" = "dylib", "windows" = "dll", "openbsd" = "so"}}}

see discussion at #275