algod: Allow timestamp offsets in dev mode

[algochoi]

Summary

This PR allows users to change timestamps in block headers in dev mode. Users can use the v2/devmode/blocks/offset algod endpoint to set a timestamp delta between blocks. There is also a small dev-mode-only change that sets the block seed in the header to the block hash (previously, the seed was not populated in dev mode).

When we are not in dev mode, the handlers return a 404 error by checking the genesis config. Optionally, we can consider adding another handler-level config like EnableDeveloperAPI to ensure that the node can accept dev-mode related APIs.

Closes #3192 #4423

Test Plan

Added dev mode handler tests and tested on sandbox using dev mode and non dev mode.

Tested that timestamp/seed related TEAL opcodes work properly in dev mode (TEAL script here):

global LatestTimestamp
block BlkSeed
block BlkTimeStamp
txn FirstValidTime

Currently can curl offset endpoint to set timestamp offsets in dev mode:

# Sandbox: Set timestamp offsets to 1000s 
curl -i -X POST -H "X-Algo-API-Token:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" localhost:4001/v2/devnet/blocks/offset/1000

[algochoi]

@jannotti I set the block seed to be the previous block's hash in dev mode as part of #4423:

// Only in dev mode
BlockHeader: BlockHeader{
	// stuff
	Seed:         committee.Seed(prev.Hash()),
},

diff: https://github.com/algorand/go-algorand/pull/5296/files#diff-da02af82e8255801f2338dd5681ec7e0ae05caf0e1a4c8ae4b34fce4b7c3613eR564

[tzaffi]

I've read a portion of the PR and am planning to read more. The approach looks solid. I left some questions and a few nit's regarding comments and documentation.

[shiqizng]

I think it can be <= here.