FTP: Add for FTPS the ability to set KeyManager and TrustManager

[TheDeadOne]

This fix adds a couple of methods to FtpsSettings to set specific KeyManager and TrustManager.

I understand that the old tag is taken as a basis, but it would be nice to release, for example, version 2.1 with this improvement.

[ennru]

Thank you for suggesting, this addition could go into Alpakka 6.0.4.
Please rebase it.

[TheDeadOne]

@ennru I hope everything is done right.

[ennru]

One more thing...

[error] akka-stream-alpakka-ftp: Failed binary compatibility check against com.lightbend.akka:akka-stream-alpakka-ftp_2.13:6.0.2! Found 1 potential problems
[error]  * method this(java.net.InetAddress,Int,akka.stream.alpakka.ftp.FtpCredentials,Boolean,Boolean,scala.Function1,scala.Option)Unit in class akka.stream.alpakka.ftp.FtpsSettings does not have a correspondent in current version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.alpakka.ftp.FtpsSettings.this")

It is fine to add an exclusion as the constructor is actually private but Mima doesn't see that.

The filter should go into a file in ftp/src/main/mima-filters/6.0.3.backwards.excludes.

[ennru]

LGTM.
Thank you!