support implicit FTPS

[dragonnn]

As far I have tested and looked into the code they isn't any support for implicit FTPs mode
https://en.wikipedia.org/wiki/FTPS#Implicit
I know it is deprecated/non-standard mode but it has some advantages and having an option for doing it would be nice.

[hannesdejager]

Thanks @dragonnn. Yes today we only support explicit FTPS.

We can look into supporting implicit if you're willing to help test. Do you have a client that you use that support this?

[dragonnn]

Yes, I am trying to test it with a Quectel BG95, it has both implicit and explicit options and I set it to implicit (and would like to keep it that way). It is doing the TLS handshake fine because I tested that with a custom TLS acceptor and that part worked fine.
I can help with testing it :)