split makefile to move it upstream

[samansmink]

The goal is to pull out-of-tree-extension.Makefile up into the duckdb repo.

That way we can just add flags and options upstream and all out-of-tree repo's will be updated.

Note that I added some vars to configure the makefile

# Inputs
#   EXT_NAME          : Upper case string describing the name of the out-of-tree extension
#   EXT_CONFIG        : Path to the extension config file specifying how to build the extension
#   EXT_FLAGS         : Extra CMake flags to pass to the build
#   EXT_RELEASE_FLAGS : Extra CMake flags to pass to the release build
#   EXT_DEBUG_FLAGS   : Extra CMake flags to pass to the debug build

Thoughts? anything missing?

Edit: oh I also remove the client tests

[carlopi]

Thanks, makes sense to me!