Adds neptune plugin for experiment tracking

[thomasjpfan]

Why are the changes needed?

This PR adds a Neptune.ai plugin for experiment tracking. This is similar to the Comet and Weights & Biases plugin.

What changes were proposed in this pull request?

This PR adds a neptune_init_run decorator that starts a Neptune Run. Given the Neptune integration with third party libraries passes the run object around, this PR injects the neptune_run object into current_context().

How was this patch tested?

This PR includes unit tests. I also ran the example in flyteorg/flytesnacks#1723:

Docs link

[

](flyteorg/flytesnacks#1723)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.73%. Comparing base (83b90fa) to head (fa6b350).
Report is 4 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2686       +/-   ##
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- Coverage   78.69%   50.73%   -27.97%     
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  Files         187      192        +5     
  Lines       19257    19425      +168     
  Branches     4029     3805      -224     
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- Hits        15155     9855     -5300     
- Misses       3404     9065     +5661     
+ Partials      698      505      -193     

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[SiddhantSadangi]

Is there any particular reason host has to be passed explicitly?
This can be extracted from the NEPTUNE_API_TOKEN

json.loads(base64.b64decode(os.getenv("NEPTUNE_API_TOKEN")).decode("utf-8"))["api_url"]

[thomasjpfan]

In a IRL conversation with @SiddhantSadangi, we discussed how this is a limitation of Flyte and dynamic log links.

Context: The Flyte UI needs to know the host URL during registration time. There are two options:

1. Have the host passed in explicitly (this PR)
2. Force users to have NEPTUNE_API_TOKEN defined during registration time.

[eapolinario]

need to update these assertions.