Fixed User.externalUsername column and creation on db

[Coduz]

This PR fixes the definition of the external_username column on usr_user table, which is incompatible with MySQL/MariaDB instances which have property innodb_large_prefix set to OFF, which limits the max length of an index to 767 bytes. See MariaDB/MySQL documentation

Related Issue
This PR fixes #3523

Description of the solution adopted
Changed the Liquibase definition for the column.

Screenshots
None

Any side note on the changes made
Added also code argument validation to the length of User.name and User.externalUsername fields.

[codecov]

Codecov Report

Merging #3583 (65ff2d1) into develop (670050c) will decrease coverage by 24.42%.
The diff coverage is 0.00%.

❗ Current head 65ff2d1 differs from pull request most recent head 2c93de4. Consider uploading reports for the commit 2c93de4 to get more accurate results

@@              Coverage Diff               @@
##             develop    #3583       +/-   ##
==============================================
- Coverage      50.23%   25.81%   -24.43%     
+ Complexity       259      115      -144     
==============================================
  Files           1662     1662               
  Lines          31940    31946        +6     
  Branches        2643     2643               
==============================================
- Hits           16045     8246     -7799     
- Misses         14958    23352     +8394     
+ Partials         937      348      -589     

Impacted Files	Coverage Δ
...e/kapua/service/user/internal/UserServiceImpl.java	3.64% <0.00%> (-47.50%)	⬇️
...se/kapua/commons/model/query/QueryFactoryImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...pse/kapua/kura/simulator/GatewayConfiguration.java	0.00% <0.00%> (-100.00%)	⬇️
...lipse/kapua/service/job/internal/JobQueryImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...lipse/kapua/service/tag/internal/TagQueryImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...kapua/transport/mqtt/MqttResponseTimeoutTimer.java	0.00% <0.00%> (-100.00%)	⬇️
...ons/configuration/ServiceConfigListResultImpl.java	0.00% <0.00%> (-100.00%)	⬇️
...a/kura/simulator/birth/BirthCertificateModule.java	0.00% <0.00%> (-100.00%)	⬇️
.../kapua/service/job/internal/JobListResultImpl.java	0.00% <0.00%> (-100.00%)	⬇️
.../kapua/service/tag/internal/TagListResultImpl.java	0.00% <0.00%> (-100.00%)	⬇️
... and 418 more