Add Kubernetes events input

[camdencheek]

Description of Changes

Adds a Kubernetes Events input operator.

Notes:

• Uses the Watch API, so no need for polling. It's all handled by the client library.
• Uses the fake library distributed with the client library to test the portions I can
• We currently require that this is run inside a Kubernetes cluster, just like with the metadata decorator. Unlike the metadata decorator, this operator could be useful outside of a k8s cluster, so it may be worth it to add client configuration in the future.
• Because we require running in a cluster, it makes it very difficult to test the Start() method. For now, testing that method is omitted.
• Adds a .dockerignore file to speed up passing in Docker build context

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	3.0173354	-0.775908	25.406385	-1.6981411
1	5000	8.241651	+0.13800526	41.768456	+1.7310104
1	10000	13.258871	-1.1207333	51.766163	-4.4341316
1	50000	57.242527	-0.34573364	188.55186	+10.09375
1	100000	109.20885	-8.0698395	300.05887	+19.68454
10	100	5.293243	+0.48280954	30.280441	+1.6879044
10	500	10.379739	-0.06878853	39.915813	-0.023033142
10	1000	15.672676	-1.5173149	53.712418	+0.29795074
10	5000	61.657585	-7.704094	163.77061	-21.083237
10	10000	119.39308	-4.09536	343.02533	+4.0599365

[codecov]

Codecov Report

Merging #88 into master will decrease coverage by 0.56%.
The diff coverage is 46.51%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   74.07%   73.51%   -0.56%     
==========================================
  Files          67       68       +1     
  Lines        4049     4133      +84     
==========================================
+ Hits         2999     3038      +39     
- Misses        794      834      +40     
- Partials      256      261       +5     

Impacted Files	Coverage Δ
operator/builtin/input/k8s_events.go	46.43% <46.43%> (ø)
...ator/builtin/transformer/k8s_metadata_decorator.go	42.64% <50.00%> (ø)

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