feat: Send + Sync trait to AsyncFtpStream/FtpStream

[newfla]

45 - Send + Sync traits to AsyncFtpStream/FtpStream

Fixes #45

Description

To make AsyncFtpStream/FtpStream Send+Sync, tls_ctx should be declared as Send+Sync. AsyncTlsConnector and TlsConnector already satisfay Send+Sync traits so no breaking change is expected

List here your changes

• tls_ctx: Option<Box<dyn AsyncTlsConnector<Stream = T>>> to tls_ctx: Option<Box<dyn AsyncTlsConnector<Stream = T> + Send + Sync + 'static>

Type of change

Please select relevant options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Acceptance tests

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• regression test: ...

[newfla]

@veeso I've missed a trait bound in both async and sync code.
Similarly, Sync trait can be added to make the use of the library much more ergonomic in multi-threaded contexts. Should I prepare a new pr or would you prefer an addition to this one?

[veeso]

@newfla No, I think you can add it here. Thank you in advance