Hardforking example

[igorcrevar]

Description

Example for EVM-607

There are two new forks:

• MyFirstFork - enabled from the block 10
• MySecondFork - enabled from the block 20

We have changed Extra structure to behave exactly like before until block 10
and to Marshall/Un-marshal/Validate one more field from block 10 until block 20
and to Marshall/Un-marshal/Validate one more field from block 20 until the end of the time

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Please complete this section if you ran manual tests for this functionality, otherwise delete it

Documentation update

Please link the documentation update PR in this section if it's present, otherwise delete it

Additional comments

Please post additional comments in this section if you have them, otherwise delete it