rpc: Add probe to capture responses from Tendermint RPC for use in testing

[thanethomson]

Closes #611 and #612.

I'm adding a small RPC probe application here that specifically does not rely on the tendermint-rpc crate and allows for maximal flexibility in JSON-RPC response structure. The idea here is to be able to execute a bunch of queries/subscriptions against a running Tendermint node and capture the responses (and possibly the requests, if needed).

These responses we can then add into our text fixtures for local "integration" testing.

The probe doesn't depend on any other crates in the repo - this is by design, since its purpose is to capture responses potentially outside the purview of our serialization infrastructure.

📖 Rendered README

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[codecov-io]

Codecov Report

Merging #653 (54cde9f) into master (0ad5f50) will decrease coverage by 0.3%.
The diff coverage is 0.1%.

@@           Coverage Diff            @@
##           master    #653     +/-   ##
========================================
- Coverage    39.5%   39.1%   -0.4%     
========================================
  Files         183     191      +8     
  Lines       12871   12550    -321     
  Branches     2943    3234    +291     
========================================
- Hits         5095    4918    -177     
+ Misses       7528    7387    -141     
+ Partials      248     245      -3     

Impacted Files	Coverage Δ
rpc-probe/src/client.rs	0.0% <0.0%> (ø)
rpc-probe/src/error.rs	0.0% <0.0%> (ø)
rpc-probe/src/kvstore.rs	0.0% <0.0%> (ø)
rpc-probe/src/plan.rs	0.0% <0.0%> (ø)
rpc-probe/src/quick.rs	0.0% <0.0%> (ø)
rpc-probe/src/request.rs	0.0% <0.0%> (ø)
rpc-probe/src/subscription.rs	0.0% <0.0%> (ø)
rpc-probe/src/utils.rs	0.0% <0.0%> (ø)
rpc-probe/src/main.rs	4.5% <4.5%> (ø)
light-client/src/errors.rs	2.3% <0.0%> (-5.6%)	⬇️
... and 14 more

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[greg-szabo]

In a follow up PR I would like to set the version number of the crate to the upcoming release and pin the Docker Hub image to the same version that the release is going to use.

[thanethomson]

In a follow up PR I would like to set the version number of the crate to the upcoming release and pin the Docker Hub image to the same version that the release is going to use.

Cool, we could do that in a separate PR or just in the v0.17.0-rc3 release PR.