Support building xcframeworks via carthage

[badeleux]

Describe the bug
Can't compile the library into xcframework due to following issue:

import RxSwift
       ^

** ARCHIVE FAILED **


The following build commands failed:
	CompileSwift normal arm64
	CompileSwiftSources normal arm64 com.apple.xcode.tools.swift.compiler
	CompileSwift normal armv7
	CompileSwiftSources normal armv7 com.apple.xcode.tools.swift.compiler
(4 failures)

To Reproduce
Steps to reproduce the behavior:

1. Try to run a carthage on a project which uses RxBluetoothKit
2. carthage bootstrap --no-use-binaries --platform iOS --use-xcframeworks

Expected behavior
A clear and concise description of what you expected to happen.

Environment:

• Device: Macbook with M1
• OS: MacOS 11.2
• Library version 6.0.0
• Swift version 5.3