Updates for DID SDK compatibility

[AlexanderShenshin]

This PR contains changes that were made for running Hedera DID SDK with this library (replacing current dependency).

Summary:

• Updated PublicKey and PrivateKey classes
  • Added from_string method for PublicKey
  • Added from_bytes method for both
    • Please note that support for loading ECDSA public key from raw bytes (encoded point) is not implemented, added TODO item for this
• Fixed protobuf conversions for Timestamp class
• Updated retry logic for client.get_transaction_receipt
  • Removed unconditional retry
    • For example, failed transactions with UNAUTHORIZED status will not be retried
  • Added more response codes for explicit "retry list" - BUSY, RECORD_NOT_FOUND, PLATFORM_NOT_ACTIVE
  • Decreased retries amount passed by transaction classes (initially was used as timeout in seconds and we we're getting large amount of retries)
• Moved transaction.get_receipt method implementation to base class
• Fixed default values of transaction memo in classes (needs to be empty string)

[AlexanderShenshin]

This change was needed to fix Windows-specific issues with timestamp parsing.

As far as I see, it should not cause any issues + tests are running fine on MacOS as well.

[nadineloepfe]

great PR, thank you so much! also for the note on ECDSA public key from raw bytes