Rfc/expose object

docs/rfcs/0001-expose-object.md

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+# Summary
+
+Provide the build facility to expose objects of nebula for outside project.
+
+# Motivation
+
+Some outside projects will use some components of nebula directly, such as CPP client will reference the `common` and `interface` code. And also useful to `UDF` and `extension` systems etc. developing in future.
+
+# Usage explanation
+
+For the module user, just try these steps:
+
+1. Include the nebula project, build the objects you need, such as `cmake --build --target base_obj` which will build the object containing all code in `base` source folder.
+2. Find the nebula as package.
+3. Include the reference headers, link with used objects. A prefix `nebula_` will be added to the object's name when exposed to outside, so user should link to `nebula_base_obj` instead of `base_obj`.
+
+# Design explanation
+
+There is a cmake macro to simplify exposing object:
+
+```cmake
+macro(nebula_add_export_library name type)
+    nebula_add_library(${name} ${type} ${ARGN})
+    export(
+        TARGETS ${name}
+        NAMESPACE "nebula_"
+        APPEND
+        FILE ${CMAKE_BINARY_DIR}/${PACKAGE_NAME}-config.cmake
+    )
+endmacro()
+```
+
+# Rationale and alternatives
+
+# Drawbacks
+
+# Prior art
+
+# Unresolved questions
+
+# Future possibilities
+
+Maybe we should provide shared library too. And the shared library could reduce the size of test binaries.