Update API to support Athena VM

[lrettig]

In #6380 the Athena VM is being integrated as an alternative VM to the genvm. However, the API still only talks to genvm. For example, ParseTransaction in the TransactionService uses genvm to parse and validate an incoming raw tx. Similarly, ParseTransaction in the v2alpha1 API uses genvm to do the same, as well as to EstimateGas. Moreover, many API tests use the genvm SDKs and templates to construct test transactions. All of this needs to be modified to support Athena, which uses a slightly different transaction format (athenavm/athena#172).

Note that the VM code in go-spacemesh currently doesn't have the ability to destructure a raw, encoded Athena TX to read its args and details such as recipient and amount (for a Spend tx). I suggest the following plan:

1. Replace all references to genvm with vm.
2. Skip all tests that use a tx type other than Spend/Spawn
3. Update tx-related services to match the Athena tx format, leaving out for now any information that requires parsing/decoding the tx
4. Discuss and decide whether we intend go-spacemesh to be able to parse/decode Athena txs, or whether we'll require callers to do this on their own

CC @poszu