Download License + scp and ftp (also e.g. for startup-config)

[steiler]

This PR makes license files downloadable and embeddable.
It further also implements scp and ftp as download protocols for also als the existing downloadable content (e.g. startup-config).

[steiler]

implementing #1377

[hellt]

I propose using sftp rather than scp
example: https://github.com/openconfig/gnmic/blob/ee2c9de75216ce19198ca5bb8f61dcc91e5da88e/utils/file.go#L128

[mabra94]

Is there any timeline when we could expect this PR to be merged? Is there anything needed to be tested/checked to get this? For Nokia internal usage this would further simplify labbing!

[steiler]

Let me base it on the actual master branch to resolve merge conflicts then @hellt can probably get to it. ;-)

[steiler]

Do we want to run the tests for the downloads as seperate suite or also make it part of smoke tests?
I feel like it is wuite some heavily lifting for whats the checked in the end.

[codecov]

Codecov Report

Attention: 202 lines in your changes are missing coverage. Please review.

Comparison is base (404e6f9) 51.78% compared to head (93bf471) 51.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1414      +/-   ##
==========================================
- Coverage   51.78%   51.19%   -0.60%     
==========================================
  Files         148      148              
  Lines       14548    14777     +229     
==========================================
+ Hits         7534     7565      +31     
- Misses       6180     6374     +194     
- Partials      834      838       +4     

Files	Coverage Δ
clab/clab.go	76.85% <100.00%> (+0.17%)	⬆️
types/topo_paths.go	68.54% <0.00%> (-1.13%)	⬇️
clab/config.go	65.94% <66.66%> (-0.64%)	⬇️
utils/file.go	35.07% <8.17%> (-19.85%)	⬇️

... and 1 file with indirect coverage changes