Add identity sign CLI command

[vaizki]

Adds a command to sign arbitrary data with the repeater / room server private key. The use is to authenticate repeater ownership on analysis platforms etc.

• data is limited to 64 bytes due to CLI input / output buffer limitations, didn't want to change them
• verification command identity verify <message> <signature> was planned but scrapped due to the same limitations
• not sure if "identity sign" is the best command name but might be an ok command namespace for future additions
• mesh::Utils::fromHex() returns 0 for errors like non-hexchars which is a bit ambiguous so a new helper parseHexBytes() was added, this could be inlined of course

Example: I want to view my node's detailed statistics so the platorm asks me to sign a random string (in this case "MeshCore"):

identity sign
  -> Usage: identity sign <data_hex>

identity sign 4d657368436f7265
  -> > E861F72FC8BBF2DBB7C4997257289CC0727D48D9A011DF6AF66F264802A1F4BF51FC864A6FF967A16AF6475A4898A23B4F8CF7F0B99A7735C3289F5C6C793802

After pasting the resulting signature to the platform it can be certain that I have access to the node and its private key, allowing me to link the repeater to my account.

Note: This function already exists in the KISS firmware as the SetHardware Extensions (0x06) -> SignData (0x04) TNC command and includes the verify command as well. So this brings the same feature to the repeater CLI.