Add option to use deterministic entropy in make_seed

[hodlwave]

I noticed that electrum does not currently have the option to generate a seed with user-provided entropy. Also, the current source of entropy comes from os.urandom() via the python-ecdsa dependency. That project's README explicitly states:

This library depends upon a strong source of random numbers. Do not use it on a system where os.urandom() does not provide cryptographically secure random numbers.

While practically speaking this will rarely be an issue, I think it's a useful to be able to supply entropy deterministically so users can eliminate this risk categorically.

My main concern with adding this feature is novices mistakenly using it and providing a very low entropy input without clearly understanding what it does. Is it correct to assume the CLI is generally only used by power-users? In any case, I made the option's description cautionary to guard against this risk.

[SomberNight]

There had already been a custom entropy option, I think for several years, but it was removed in 5e5134b
Though note that that mixed in user-provided entropy, unlike your proposal which uses it exclusively.

I think a feature that relies entirely on user-provided entropy is dangerously easy to misuse: it's a footgun.

[SomberNight]

Related:
#523
#1574
#1964
#2070

[hodlwave]

Good points. Come to think of it, adding user entropy with a strong passphrase should be sufficient for this purpose anyway.