enhancement: disallow Register() name dupes

[tzaffi]

Summary

Proposing that when Register() is called twice for the same plugin name, it will panic.
An alternative would involve changing the signature to return an error.

Explanation

This is mostly about familiarizing myself with the Conduit repo. This is not an urgent issue.

As I was playing around with conduit plugin templates I realized there is a slight issue with plugin registration.

It's possible to register a plugin with the same name as another, and overwrite the first registration

The current functionality keeps a map from a user provided name to the actual plugin. If a plugin with the same name as another is registed a second time, it simply overwrites the first. This certainly isn't a pressing concern as we have complete control over what to register and what not to.

Test Plan

CI

[codecov]

Codecov Report

Merging #72 (5ec7257) into master (442791a) will increase coverage by 0.92%.
The diff coverage is 73.86%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   67.66%   68.58%   +0.92%     
==========================================
  Files          32       36       +4     
  Lines        1976     2394     +418     
==========================================
+ Hits         1337     1642     +305     
- Misses        570      660      +90     
- Partials       69       92      +23     

Impacted Files	Coverage Δ
conduit/data/block_export_data.go	100.00% <ø> (+92.30%)	⬆️
conduit/plugins/importers/algod/metrics.go	100.00% <ø> (ø)
pkg/cli/internal/list/list.go	20.75% <ø> (ø)
...lugins/exporters/postgresql/postgresql_exporter.go	66.66% <51.21%> (-11.54%)	⬇️
pkg/cli/cli.go	65.97% <65.97%> (ø)
conduit/pipeline/pipeline.go	66.09% <72.10%> (+0.64%)	⬆️
conduit/data/config.go	76.47% <76.47%> (ø)
conduit/plugins/importers/algod/algod_importer.go	84.69% <88.29%> (-3.62%)	⬇️
conduit/pipeline/errors.go	100.00% <100.00%> (ø)
conduit/plugins/exporters/exporter_factory.go	100.00% <100.00%> (ø)
... and 3 more

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[winder]

Good idea, nice catch.