Make it easy for projects to depend on libxrpl

[thejohnfreeman]

This changeset does a few things:

• Adds an ALIAS target named xrpl::libxrpl for projects to link.
  • I chose this name for a few reasons. 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. (I think underscores in target or library names are generally discouraged anyway.) Every target exported in CMake should be prefixed with the project name. The project should not be named after the company Ripple. I think most of us prefer to just name it after the ledger.
  • By adding an ALIAS target, existing consumers who use the xrpl_core target will not be affected. In the future, we can start 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.
• Fixes the Conan recipe so that consumers using Conan import a target named xrpl::libxrpl. This way, every consumer can use the same instructions.
• Documents the two easiest methods to depend on libxrpl. I have tested both. I'll work on adding CI tests for both methods.

See #4443.

[seelabs]

Both the build system changes look good and I liked the doc as well. Nice job on this!

Note that I did not create a project to exercise the instructions in the doc. I'll try to find time to do so, but this is a low risk change. I think it's ok to merge it. 👍 from me.

[mvadari]

I don't know CMake well enough to review changes so I'm not going to formally approve this, but I'm glad this is happening and this will be really useful.

[intelliot]

@thejohnfreeman if you agree this is ready to merge, you can put the Passed label on it, or just comment saying that it's good to go