common: add unified HSM secret handling module

This is a newer API to replace hsm_encryption.c and hsm_encryption.c, this tidies up the API to be used and also cleans things up to support our new formts. Our hsm_secret formats now include:
- Legacy 32-byte plain format
- Legacy 73-byte encrypted format
- New mnemonic format without passphrase (32 zero bytes + mnemonic)
- New mnemonic format with passphrase (32-byte hash + mnemonic)

This commit includes support to detect the format based on the file size and content structure. The hsm will store mnemonics in the hsm_secret file as:

`passphraseHash`mnemonic`

Author: sangbida

common/Makefile

@@ -46,6 +46,7 @@ COMMON_SRC_NOGEN :=				\
 	common/hmac.c				\
 	common/hsm_capable.c			\
 	common/hsm_encryption.c			\
+	common/hsm_secret.c			\
 	common/htlc_state.c			\
 	common/htlc_trim.c			\
 	common/htlc_tx.c			\