Add custom thor solo block interval

[MakisChristou]

Tackles the following issue. In this PR I basically added the option to use a command-line argument specifically for solo to allow the developer to choose a custom block interval with a minimum value of 1. I tested it and it looks like its working fine for 1 second block intervals as well as larger intervals like 100 seconds.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 61.33%. Comparing base (88baecd) to head (ca026c6).

Files	Patch %	Lines
cmd/thor/solo/solo.go	0.00%	6 Missing :warning:
cmd/thor/main.go	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
- Coverage   61.34%   61.33%   -0.02%     
==========================================
  Files         194      194              
  Lines       18221    18224       +3     
==========================================
  Hits        11177    11177              
- Misses       5961     5965       +4     
+ Partials     1083     1082       -1     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[libotony]

Some changes has been made to solo, please update accordingly, check PR https://github.com/vechain/thor/pull/722 @MakisChristou