Add CLI random padding support for SPA packets (fixed value + random range)

[N0TScrooge]

Hello, and thank you for maintaining fwknop.

I’d like to propose a new client feature: add padding to SPA packets from the command line, so packet length can be controlled/varied and become less predictable on the wire.
Proposed functionality

Please add two CLI options (names are examples):

--pad-bytes <N>
Add a fixed number of padding bytes to the SPA payload.

--pad-random <MIN:MAX>
For each SPA packet, choose a random padding length in the [MIN, MAX] range.

Example usage

fwknop ... --pad-bytes 64
fwknop ... --pad-random 16:128

Expected behavior

Padding must be part of the encrypted/authenticated SPA content (not appended after ciphertext).
Default behavior remains unchanged (padding disabled unless requested).
Input validation:
    MIN <= MAX
    enforce a sane upper bound (e.g., do not exceed MAX_SPA_ENCRYPTED_SIZE/MTU constraints).
With --pad-random, a new random length is chosen per packet.
Works consistently across UDP/TCP/HTTP SPA transport modes.

Why this is useful

Reduces SPA traffic correlation based on packet length.
Makes signature/heuristic traffic analysis harder.
Gives users a controlled way to vary packet length without changing access semantics.

Optional follow-ups

Add corresponding settings to ~/.fwknoprc (e.g., PAD_BYTES / PAD_RANDOM).
Add unit/integration tests for:
    CLI parsing,
    random range validation,
    successful server-side processing of padded packets.

Thanks!