Add Pantheon JSON-RPC eth_chainId method

[lucassaldanha]

What does this PR do?

Recently we implemented EIP-695 on Pantheon (Pantheon PR link).

In this PR, we are introducing Pantheon module with the factory for the Pantheon object that contains methods implemented by Pantheon that haven't made their way into the main JSON-RPC spec.

Where should the reviewer start?

Probably start by reviewing the tests to understand the expected behaviour. After that, review the plumbing code to ensure I didn't miss something.

Why is it needed?

As we add methods to Pantheon JSON-RPC API, it would be great to have them available on Web3j so users can keep using the library to interact with Pantheon nodes.
We are about to release Pantheon nodes running on Infura so it would be great to make things easier for the users.

[codecov]

Codecov Report

Merging #766 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #766      +/-   ##
============================================
- Coverage     77.38%   77.26%   -0.13%     
- Complexity     1851     1854       +3     
============================================
  Files           241      244       +3     
  Lines          6814     6821       +7     
  Branches       1012     1012              
============================================
- Hits           5273     5270       -3     
- Misses         1291     1301      +10     
  Partials        250      250

Impacted Files	Coverage Δ	Complexity Δ
...rg/web3j/protocol/pantheon/JsonRpc2_0Pantheon.java	100% <100%> (ø)	2 <2> (?)
.../pantheon/methods/response/EthChainIdResponse.java	100% <100%> (ø)	2 <2> (?)
...ain/java/org/web3j/protocol/pantheon/Pantheon.java	100% <100%> (ø)	1 <1> (?)
...n/java/org/web3j/protocol/core/filters/Filter.java	53.12% <0%> (-14.07%)	8% <0%> (-2%)
...org/web3j/protocol/websocket/WebSocketService.java	85.26% <0%> (-1.06%)	50% <0%> (-1%)
core/src/main/java/org/web3j/utils/Flowables.java	88% <0%> (+4%)	11% <0%> (+1%)	⬆️

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[lucassaldanha]

I'm closing this PR to let others work on this during the hackathon. I'm happy to reopen it later to integrate other Pantheon methods on Web3j.

[iikirilov]

Work was started in the hackathon #767

[fcorneli]

Seems like eth_chainId is also supported by both geth and parity. This could live within core I guess...
See also:
e-Contract/ethereum-resource-adapter@1d3d060