feat: support TX type CONTEND_SLOTS & RELEASE_SLOTS

[josephzxy]

Description

Currently, VSYS has 10 TX types as shown below.

class TxType(enum.Enum):
    """
    TxType is the enum class for transaction types
    """

    GENESIS = 1
    PAYMENT = 2
    LEASE = 3
    LEASE_CANCEL = 4
    MINTING = 5
    CONTEND_SLOTS = 6
    RELEASE_SLOTS = 7
    REGISTER_CONTRACT = 8
    EXECUTE_CONTRACT_FUNCTION = 9
    DB_PUT = 10

For py-v-sdk, TX types below are not required to support.

• GENSIS
• MINTING

As of de5ceda, below Tx types are not yet supported.

• CONTEND_SLOTS
• RELEASE_SLOTS

We need to support them but the priority is relatively low as they are nice to have.