Java: Seed & indexable test

[kwek20]

Description of change

• Fixed the pom example to run from root or child
• Added tests for seed usage
• Added tests for indexable usage
• Made Payloads and Message serialisable
• Fixed MQTT(temp fix: Java: Sync callback Dushistov/flapigen-rs#388)

Links to any relevant issues

Fixes #790

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Documentation Fix

How the change has been tested

Ran the new tests, also verified with a non java made indexable

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[thibault-martinez]

Is this ready to be reviewed/merged ?

[kwek20]

@thibault-martinez Yessir!

[kwek20]

@Thoralf-M

[kwek20]

@thibault-martinez we can merge!