build: add interface library libxrpl: (#4449)

Make it easy for projects to depend on libxrpl by adding an `ALIAS`
target named `xrpl::libxrpl` for projects to link.

The name was chosen because:

* The current library target is named `xrpl_core`. There is no other
  "non-core" library target against which we need to distinguish the
  "core" library. We only export one library target, and it should just
  be named after the project to keep things simple and predictable.
* Underscores in target or library names are generally discouraged.
* Every target exported in CMake should be prefixed with the project
  name.

By adding an `ALIAS` target, existing consumers who use the `xrpl_core`
target will not be affected.

* In the future, there can be a migration plan to make `xrpl_core` the
  `ALIAS` target (and `libxrpl` the "real" target, which will affect the
  filename of the compiled binary), and eventually remove it entirely.

Also:

* Fix the Conan recipe so that consumers using Conan import a target
  named `xrpl::libxrpl`. This way, every consumer can use the same
  instructions.
* Document the two easiest methods to depend on libxrpl. Both have been
  tested.
* See #4443.

Builds/CMake/RippledCore.cmake

@@ -13,6 +13,10 @@ if (unity)
   set_target_properties(xrpl_core PROPERTIES UNITY_BUILD ON)
 endif ()
 
+add_library(libxrpl INTERFACE)
+target_link_libraries(libxrpl INTERFACE xrpl_core)
+add_library(xrpl::libxrpl ALIAS libxrpl)
+
 
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     beast/legacy FILES: