feat: add txpool namespace RPC methods

[crypto-facs]

Closes: #124

Description

Enabled txpool namespace with txpool_content method that complies with the go ethereum documentation. The decision was to return an empty response (this is explained in further detail in the issue). Endpoint is needed to unblock block explorer integration

NOTE: Did not update documentation as the method is not fully functional yet. Happy to do it if it is required.

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[fedekunze]

Looks good! Can you also add the txpool_inspect and txpool_status endpoints to return empty values? See https://geth.ethereum.org/docs/rpc/ns-txpool

[crypto-facs]

Hey @fedekunze thanks for the review! Just added the other two endpoints with a similar approach. Feel free to give it a final look!

[fedekunze]

ACK. Can you also add a description on de CHANGELOG.md with the issue number and description under the Unreleased > Improvements section?

[fedekunze]

follow up issue, add tests for RPC and documentation of the RPC endpoint

[codecov]

Codecov Report

Merging #146 (acd394f) into main (6be18a1) will increase coverage by 0.85%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   68.81%   69.67%   +0.85%     
==========================================
  Files           5        5              
  Lines         635      643       +8     
==========================================
+ Hits          437      448      +11     
+ Misses        183      179       -4     
- Partials       15       16       +1     

Impacted Files	Coverage Δ
app/ante/eth.go	88.54% <0.00%> (-0.55%)	⬇️
app/ante/ante.go	46.37% <0.00%> (+8.14%)	⬆️