Update network config to be more usable

[yknl]

This PR adds the following methods to the StacksNetwork interface:

getBroadcastApiUrl: () => string;
getTransferFeeEstimateApiUrl: () => string;
getAccountApiUrl: (address: string) => string;
getAbiApiUrl: (address: string, contract: string) => string;

And renames the API endpoints:

broadcastEndpoint: string;
transferFeeEstimateEndpoint: string;
accountEndpoint: string;
contractAbiEndpoint: string;

This update makes it possible to create custom network configs by overriding the coreApiUrl property of the existing StacksMainnet or StacksTestnet configs.

Type of Change

• [x] New feature
• [ ] Bug fix
• [ ] API reference/documentation update
• [ ] Other

Does this introduce a breaking change?

No

Are documentation updates required?

No

Testing information

Checklist

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

[codecov[bot]]

Codecov Report

Merging #81 into abi will increase coverage by 0.03%. The diff coverage is 77.27%.

@@            Coverage Diff             @@
##              abi      #81      +/-   ##
==========================================
+ Coverage   83.83%   83.87%   +0.03%     
==========================================
  Files          27       27              
  Lines        1652     1656       +4     
  Branches      348      349       +1     
==========================================
+ Hits         1385     1389       +4     
  Misses        265      265              
  Partials        2        2              

Impacted Files	Coverage Δ
src/builders.ts	72.86% <58.33%> (+0.04%)	:arrow_up:
src/network.ts	100.00% <100.00%> (+5.26%)	:arrow_up:

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