enhance: remote slack attachment

[claire1618]

These changes allows to Slack could source a message file from another repo.

This will help is someone has to send a message held in one repo across multiple repos.

Below is an example for the new parameters introduced (remote, registry):

parameters:
    filepath: $remote-filepath
    remote: true
    registry: https://github.com

[codecov]

Codecov Report

Merging #56 (03874b5) into main (1213574) will increase coverage by 4.76%.
The diff coverage is 68.96%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   17.94%   22.70%   +4.76%     
==========================================
  Files           2        2              
  Lines         574      643      +69     
==========================================
+ Hits          103      146      +43     
- Misses        463      488      +25     
- Partials        8        9       +1     

Files	Coverage Δ
cmd/vela-slack/plugin.go	84.39% <95.23%> (+3.29%)	⬆️
cmd/vela-slack/main.go	0.00% <0.00%> (ø)

[wass3rw3rk]

we can probably fix the couple of linter errors in main.go. we got rid of io/ioutil elsewhere for example, we can use the correct module in main.

[ecrupper]

Can we get these updates to DOCS.md since we pull those in for our doc site? Maybe in a separate PR so it isn't updated prematurely?

[ecrupper]

neat