fix: Specify transitive_headers when building with Conan 2 (#5462)

To be able to consume `rippled` in Conan 2, the recipe should specify transitive_headers for external libraries that are present in the exported header files. This change remains compatibility with Conan 1, where this flag was not present.

conanfile.py

@@ -1,4 +1,4 @@
-from conan import ConanFile
+from conan import ConanFile, __version__ as conan_version
 from conan.tools.cmake import CMake, CMakeToolchain, cmake_layout
 import re
 
@@ -24,13 +24,11 @@ class Xrpl(ConanFile):
     }
 
     requires = [
-        'date/3.0.3',
         'grpc/1.50.1',
         'libarchive/3.7.6',
         'nudb/2.0.8',
         'openssl/1.1.1v',
         'soci/4.0.3',
-        'xxhash/0.8.2',
         'zlib/1.3.1',
     ]
 
@@ -99,7 +97,10 @@ class Xrpl(ConanFile):
             self.options['boost'].visibility = 'global'
 
     def requirements(self):
-        self.requires('boost/1.83.0', force=True)
+        # Conan 2 requires transitive headers to be specified
+        transitive_headers_opt = {'transitive_headers': True} if conan_version.split('.')[0] == '2' else {}
+        self.requires('boost/1.83.0', force=True, **transitive_headers_opt)
+        self.requires('date/3.0.3', **transitive_headers_opt)
         self.requires('lz4/1.10.0', force=True)
         self.requires('protobuf/3.21.9', force=True)
         self.requires('sqlite3/3.47.0', force=True)
@@ -107,6 +108,7 @@ class Xrpl(ConanFile):
             self.requires('jemalloc/5.3.0')
         if self.options.rocksdb:
             self.requires('rocksdb/9.7.3')
+        self.requires('xxhash/0.8.2', **transitive_headers_opt)
 
     exports_sources = (
         'CMakeLists.txt',