server: support PASV response with custom address

[janneronkko]

This allows old FTP client to connect to AIOFtp server located behind NAT.

What do these changes do?

The change adds support for clients using passive connections (PASV command) to a server located behind NAT

Are there changes in behavior for the user?

Passive connections can be opened with PASV command to server behind (IPv4) NAT

Related issue number

N/A

Checklist

• [x ] I think the code is well written
• [x ] Unit tests for the changes exist
• [x ] Documentation reflects the changes
• [ ] Add a new news fragment into the CHANGES folder -- there is no CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
  • .feature: Signifying a new feature.
  • .bugfix: Signifying a bug fix.
  • .doc: Signifying a documentation improvement.
  • .removal: Signifying a deprecation or removal of public API.
  • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[janneronkko]

I faced this issue with a service I've implemented; all the clients connecting to our service do not use EPSV command (using EPSV would have worked)

[codecov[bot]]

Codecov Report

Merging #150 (0a92e16) into master (ad28ae9) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   99.15%   99.15%           
=======================================
  Files           6        6           
  Lines        1888     1891    +3     
=======================================
+ Hits         1872     1875    +3     
  Misses         16       16           

Impacted Files	Coverage Δ
aioftp/server.py	98.38% <100.00%> (+<0.01%)	:arrow_up:

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[pohmelie]

Thank you for the PR, released as 0.21.0.