feat: 'addSignature' method to StacksTransaction, closes #121

[kyranjamie]

Example: As a Blockstack developer, I must be able to interact with an API exposed by StacksTransaction to add a signature to an unsigned transaction.

This change is implemented with an immutable api.

Is there a case in which spending condition would be undefined? Would be a lot cleaner if a tx were to always have one.

const unsignedTx = makeUnsignedSTXTokenTransfer(txOptions);
const signedTx = unsignedTx.setSignature(sig);

See issue #121

Checklist

• [x] Code is commented where needed
• [x] Unit test coverage for new or modified code paths
• [x] npm run test passes
• [x] Changelog is updated
• [x] Tag 1 of @yknl, @zone117x, @reedrosenbluth for review

[codecov[bot]]

Codecov Report

Merging #123 into master will increase coverage by 0.00%. The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master     #123   +/-   ##
=======================================
  Coverage   83.30%   83.31%           
=======================================
  Files          28       28           
  Lines        2156     2163    +7     
  Branches      444      446    +2     
=======================================
+ Hits         1796     1802    +6     
- Misses        356      357    +1     
  Partials        4        4           

Impacted Files	Coverage Δ
src/transaction.ts	79.64% <85.71%> (+0.40%)	:arrow_up:

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[reedrosenbluth]

I'm not sure I understand the use case here, where would the signature come from? Why not use the TransactionSigner as per usual? How would this work for Multi-sig?

Is there a case in which spending condition would be undefined?

I can't think of one.

[kyranjamie]

The API we have to work with a Ledger device is a essentially a function with the following type:

(unsignedSerialisedTransaction: string) => Buffer // of the signature created by the device

This PR adds a simple method to add that signature to the unsigned transaction and add a new one.

I'm not sure how this would be achieved with the TransactionSigner currently. It only accepts a StacksTransaction.