Replace Bouncy Castle dependency with in-project CMAC implementation

[coderabbitai]

The Bouncy Castle dependency was added as a temporary stopgap for proving a CMAC implementation. Replace it with an in-project CMAC implementation to reduce external dependencies.

Context:

• PR: Implement message signing #2
• Source comment: Implement message signing #2 (comment)
• See also: Implement message signing #2 (comment)
• Requested by: @JaniruTEC (Comment: Implement message signing #2 (comment))

[overheadhunter]

see https://github.com/cryptomator/siv-mode/blob/3841573c8dcdc74571da848921c72cffa645f3ff/src/main/java/org/cryptomator/siv/CMac.java