Add port to SCP Endpoints

[balkian]

The port for SCP-like URLs was hardcoded to 22.
This commit modifies the regex to find a port (optional), and adds a new test case that covers this scenario.

All tests pass, and I've tried cloning a repo with a custom port through the command line example and it works.

Some notes:

• 22 is kept as the default port. The ssh transport also has a default port defined . It may be best to use 0 and let the ssh transport handle it.
• I use strconv.Atoi in the getter method to mimic the behavior of the other types. It might be more efficient to just parse it once after the regex.

[codecov]

Codecov Report

Merging #608 into master will decrease coverage by 0.57%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #608      +/-   ##
==========================================
- Coverage   78.31%   77.74%   -0.58%     
==========================================
  Files         130      130              
  Lines       10135    10140       +5     
==========================================
- Hits         7937     7883      -54     
- Misses       1347     1419      +72     
+ Partials      851      838      -13

Impacted Files	Coverage Δ
plumbing/transport/common.go	92.4% <100%> (+0.51%)	⬆️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️

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[mcuadros]

Awesome tanks!