Support selective plugin builds by passing an array of plugins to `make`

[ShahanaFarooqui]

Currently, make builds all plugins defined in Cargo.toml’s workspace, even when only a subset is needed. This increases build time unnecessarily, especially for users who only require specific plugins.

We can pass an argument to make that accepts a space-separated list of plugins to compile, skipping unnecessary ones.

Example Usage:

# Build only grpc and rest plugins
make RUST_PLUGINS="cln-grpc-plugin clnrest"

# Default: build all plugins if none specified
make

Reference:
By "Hosted Marcellus" on Discord:

Recent versions of CLN take a very long time to build in my VPS due to the Rust binaries. 
Of these I'm only really interested in cln-rest, is there any way to tell the build process to 
not try to build the others?  

Currently I'm just doing "make RUST_PROFILE=release"