Expose APIs for working with transaction proposals

[str4d]

Closes Electric-Coin-Company/zcash-swift-wallet-sdk#1343.

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[str4d]

Force-pushed to address review comments.

[str4d]

Force-pushed to clean up the commits.

[LukasKorba]

I tested all 5 APIs while working on adoption of this PR (e9177a28f73d19f81849108e9b4978a10a08a582) in Zashi. I can confirm

• deprecated Send API still works ok
• deprecated shileding API still works ok
• new proposeTransfer API works ok
• new proposeShielding API works ok
• createProposedTransactions API works ok

[str4d]

Before we merge this, I'm going to add a helper method to Proposal to expose the number of steps, so that callers can anticipate how many events they will get from the async stream returned from createProposedTransactions.

[str4d]

Force-pushed to remove the 2.0.11 changelog bump, as @LukasKorba will make a few other release updates before the actual release commit.