boost: build extra boost_pythonXX libs, add python_executableXX options

Boost b2 supports building Boost.Python for multiple python versions.
(https://www.boost.org/doc/libs/1_79_0/libs/python/doc/html/building/configuring_boost_build.html)

Pass option `boost:pythonXY_executable` to enable building
boost_python for X.Y of python.

For example when in a python 3.10 environment, adding these options:

    -o boost:python27_executable=`which python2.7` \
    -o boost:python36_executable=`which python3.6` \
    ...

Will build libboost_python310 (the default), libboost_python27 and libboost_python36.

recipes/boost/all/conanfile.py

@@ -57,6 +57,8 @@
     "wave",
 )
 
+PYTHON_VERSIONS = ("2.7", "3.5", "3.6", "3.7", "3.8", "3.9", "3.10")
+
 
 class BoostConan(ConanFile):
     name = "boost"
@@ -105,6 +107,8 @@ class BoostConan(ConanFile):
         "system_use_utf8": [True, False],
     }
     options.update({"without_{}".format(_name): [True, False] for _name in CONFIGURE_OPTIONS})
+    # extra boost_python libs to build
+    options.update({"python{}_executable".format(_name.replace('.', '')): "ANY" for _name in PYTHON_VERSIONS})
 
     default_options = {
         "shared": False,
@@ -143,6 +147,8 @@ class BoostConan(ConanFile):
     }
     default_options.update({"without_{}".format(_name): False for _name in CONFIGURE_OPTIONS})
     default_options.update({"without_{}".format(_name): True for _name in ("graph_parallel", "mpi", "python")})
+    # extra boost_python libs to build
+    default_options.update({"python{}_executable".format(_name.replace('.', '')): "None" for _name in PYTHON_VERSIONS})
 
     short_paths = True
     no_copy_source = True
@@ -1072,6 +1078,11 @@ def create_python_config(python_tool):
 
             contents += create_python_config(self._python)
 
+            for version in PYTHON_VERSIONS:
+                option = self.options.get_safe("python{}_executable".format(version.replace('.', '')))
+                if option:
+                    contents += create_python_config(PythonTool(version, option.value, self))
+
         if not self.options.without_mpi:
             # https://www.boost.org/doc/libs/1_72_0/doc/html/mpi/getting_started.html
             contents += "\nusing mpi ;"