Add XcodeToolchain.build_settings for settings without a dedicated conf

[czoido]

Changelog: Feature: XcodeToolchain now accepts arbitrary extra xcconfig settings via tc.build_settings, for build settings with no dedicated conf.
Docs: conan-io/docs#4524

Named .build_settings that is the official name Apple gives to this variables: https://developer.apple.com/documentation/xcode/build-settings-reference

There was no way for a recipe to set an Xcode-only build setting through XcodeToolchain, the way e.g. CMakeToolchain.variables lets you set arbitrary CMake variables, for example OTHER_SWIFT_FLAGS.

This adds self.build_settings, a plain dict a recipe can populate before calling tc.generate():

tc = XcodeToolchain(self)
tc.build_settings["OTHER_SWIFT_FLAGS"] = "$(inherited) -cxx-interoperability-mode=default ..."
tc.generate()

[AbrilRBS]

Needs rebasing, but the extra xxconfig bits look good :)