Add Prepare and Process Proposal ABCI++ methods

[evan-forbes]

Description

This PR incorporates the two needed ABCI++ methods from upstream.

• cherry-picks the PrepareProposal and ProcessProposal ABCI++ methods
• Modifies both methods to work on tendermint v0.34.x
  • This includes minor changes such as removing context usuage and returning errors, along with moving code outside of interanlized packages.
• Modifies both methods to use types.Data instead of types.Txs or [][]byte

Closes #623

related to #157

[evan-forbes]

Still unsure as to why the test_apps script is failing. We've had trouble with it in the past, so I'm not convinced that it is a deal breaker for merging this PR as long as both the e2e and app's integration test are passing. This test will likely be fixed when we use a stable version of ABCI++.

[evan-forbes]

tbc, this PR is a placeholder that allows us to start building features in the app that depend upon these two added methods. When the release version of the cosmos-sdk that uses tendermint v0.36.x comes out, then we can create a version of celestia-core that is based on v0.36.x and use that instead

[liamsi]

This is pretty cool! Looking forward to testing this with the SDK/our app.

[liamsi]

Left one nit. The changes here are straightforwardly pulling from upstream.

I think it would be important to clarify in the opening comment from which branch(es)/commit(s)/version(s) this was pulled exactly.

Similarly:

Modifies both methods to work on tendermint v0.34.x

It would be good if the PR opening comment documented what modifications were necessary here.