Cherry-pick #24913 to 7.x: Add kubernetes_leaderelection provider

[ChrsMark]

Cherry-pick of PR #24913 to 7.x branch. Original message:

What does this PR do?

Adds support for leaderelection provider in Agent based on #24267.

After this one is merged I plan to tune standalone manifest in follow-up PR so as to remove deployment section and only deploy as Deamonset with leaderelection "on".

Why is it important?

To support cluster wide metrics collection by only one Agent at a time between a group of multiple Agents (ie Deamonset).

On a second step we can change the standalone manifest, completely remove the Deployment of Agent and go on with only the Daemonset leveraging the leaderelection feature.

How to test this PR locally

1. Enable the provider and an input based on the leader condition:

providers:
  kubernetes_leaderelection:
    kube_config: /Users/chrismark/.kube/config
    leader_lease: elastic-agent
  
inputs:
  - type: logfile
    condition: ${kubernetes_leaderelection.leader} == true
    streams:
      - paths: /var/log/leaderlog/another.log

2. run inspect command to verify the output: ./elastic-agent -e -d "*" -c /elastic-agent.yml inspect output -o default
3. Verify the conditional input is enabled:

filebeat:
  inputs:
  - index: logs-generic-default
    paths: /var/log/leaderlog/another.log
    processors:
    - add_fields:
        fields:
          dataset: generic
          namespace: default
          type: logs
        target: data_stream
    - add_fields:
        fields:
          dataset: generic
        target: event
    - add_fields:
        fields:
          id: 32806c0a-4f04-499c-9427-e6e24e5f6035
          snapshot: false
          version: 8.0.0
        target: elastic_agent
    type: log

4. (Optional:) Perform same test from different Agents and on a running mode so as to verify that only one of them picks the lock and becomes the leader

Related issues

• Closes [Agent] Implement leader election on k8s for Elastic Agent #24267

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Pinging @elastic/agent (Team:Agent)

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[elasticmachine]

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• Build Cause: Started by user Chris Mark

• Start Time: 2021-04-08T11:45:55.942+0000

• Duration: 57 min 10 sec

• Commit: f467459

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Skipped	24
Total	6588

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Test	Results
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Passed	6564
Skipped	24
Total	6588