Active transfer local IP resolver

[syncplify]

This change fills a gap in the library, specifically on multi homed hosts (hosts with multiple NICs, or NICs with multiple IP addresses) where the library didn't have the capability to initiate the outbound connection to the client, when in active FTP mode, from the same IP address as the inbound connection it's responding to.
For flexibility this adds an optional resolver in the driver, to let developers implement their own resolution logic. This will typically be "same as binding" but this design doesn't restrict the developer to only that one option.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.47%. Comparing base (e6a2418) to head (900227d).

Files with missing lines	Patch %	Lines
transfer_active.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #674      +/-   ##
==========================================
- Coverage   87.48%   87.47%   -0.02%     
==========================================
  Files          13       13              
  Lines        2102     2108       +6     
==========================================
+ Hits         1839     1844       +5     
  Misses        175      175              
- Partials       88       89       +1     

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