Cherry-pick of PR #24789 to 7.x branch. Original message:

This PR aims to add support for consuming k8s secrets through Agent's variables.

Note: This is still a draft but comments are needed so as to verify it's going to the right direction, so plz @blakerouse @exekias feel free to comment here.

How to test this:

0. Create the k8s secret:

cat << EOF | kubectl apply -f -apiVersion: v1
kind: Secret
metadata:
  name: somesecret
type: Opaque
data:
  value: $(echo -n "passpass" | base64)
EOF

1. Use the following config:

providers.kubernetes_secrets:
  kube_config: /Users/chrismark/.kube/config

inputs:
  - type: logfile
    streams:
      - paths: ${env.logpath}/${kubernetes_secrets.default.somesecret.value}/another.log

2. Run inspect command to check what is the compiled configuration (use proper path for the elastic-agent.yml config file :
   logpath=/var/log ./elastic-agent -c /Users/ubuntu/Desktop/elastic-agent.yml inspect output -o default

Verify that the following is produced:

[default] filebeat:
filebeat:
  inputs:
  - index: logs-generic-default
    paths: /var/log/passpass/another.log

Unit testing:

1. go test -v ./pkg/agent/transpiler/... -run TestVars_ReplaceWithFetchContextProvider
2. go test -v ./pkg/composable/providers/kubernetessecrets...

Closes #24143