algod: Modify simulate endpoint request type

[jasonpaulos]

Summary

Currently the /v2/transactions/simulate endpoint has the same request format as /v2/transactions, the transaction submission endpoint. This format is a concatenated byte array of all of the encoded transactions in an atomic group. Unfortunately this format has the following shortcomings for simulate:

• Only a single atomic group can be expressed (without assuming correct group IDs)
• There is no space for additional parameters, which would be required for configuring the simulation environment

To solve these problems, this PR creates a new SimulationRequest object that serves as the /v2/transactions/simulate endpoint's request type. This object can be encoded using either JSON or msgpack. At the moment this object does not offer any additional features, but the goal is to introduce new fields in the future for this purpose.

NOTE: this new request object is a backwards-incompatible change. Normally we would not advocate for this, but since simulate is such a new feature and this change would vastly improve our ability to add related features, we think this is a tradeoff that makes sense.

If you already have a transaction group formatted for /v2/transactions or the old version of /v2/transactions/simulate prior to this PR, you can use the new flag --request-only-out for goal clerk simulate to create a properly-formed simulate request:

goal clerk simulate -t <input_txn_group_file> --request-only-out <request_output_file>

The --request-only-out flag indicates that you want goal to help you format a simulate request. It will create the request and write it to the specified file, then exit. No simulation will happen.

Test Plan

Modified current tests and added additional tests

[ahangsu]

I am generally good on this, maybe a few details we want to be clear for following steps:

1. unlimited-log should go under simulate/extended
2. minor questions on erroring early in SimulateTxnExtended handler

[ahangsu]

generally lookin good, and testing looks nice!

[ahangsu]

There are a few things to be fixed in clerk.go, but others should be fine.