Add Pod object to K8s meta

[DmitryRomanov]

Description

Add k8s.io/api/core/v1.Pod object to meta

Example of use

input:
  type: k8s
  file_config:
    persistence_mode: async
    filename_pattern: "*"
  watching_dir: /var/log/containers/
  offsets_file: /data/k8s-offsets.yaml
  allowed_pod_labels: [app, jobid]
  meta:
    component: "{{ index .pod.Labels \"component\" }}"

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 20 lines in your changes missing coverage. Please review.

Project coverage is 72.98%. Comparing base (e02c4a6) to head (d558c90).
Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
plugin/input/k8s/meta/gatherer.go	72.97%	10 Missing ⚠️
plugin/input/k8s/meta/meta.go	20.00%	7 Missing and 1 partial ⚠️
pipeline/metadata/templater.go	95.74%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #710      +/-   ##
==========================================
+ Coverage   72.28%   72.98%   +0.70%     
==========================================
  Files         137      142       +5     
  Lines       13985    14633     +648     
==========================================
+ Hits        10109    10680     +571     
- Misses       3311     3359      +48     
- Partials      565      594      +29     

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