feat: add FreezeTransaction

[Dosik13]

Description:
Implemented the FreezeTransaction class and FreezeType enum that allow freezing network.

• Add FreezeType enum class implementation with support for all freeze types (FREEZE_ONLY, PREPARE_UPGRADE, FREEZE_UPGRADE, FREEZE_ABORT, TELEMETRY_UPGRADE)
• Add FreezeTransaction class implementation that enables freezing the network with configurable parameters (start_time, file_id, file_hash, freeze_type)
• Add unit tests for FreezeTransaction class
• Add unit tests for FreezeType enum class
• Add FreezeTransaction and FreezeType to __init__.py file in __all__ list
• Update CHANGELOG with FreezeTransaction and FreezeType functionality additions

Related issue(s):

Fixes #260

Notes for reviewer:

Didn't add integration tests as it is hard and unnecessary to test. (I believe it won't be used by the users)

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[nadineloepfe]

awesome PR, thank you! Would it be possible to also add an example script thought (hardcoded to solo ofc? just for documentation purposes with a warning about required permissions. I'm fine with not adding integration tests however

[Dosik13]

Won't there be a problem with the CI, because when it runs the example it could stop the network 😄

[nadineloepfe]

@Dosik13: that's a fair point. would need an unfreeze. ok, changelog rebase and we're good, thanks :D