refactor: simplify alloy installation by only requiring k8s secrets

[brunodam]

Description

This pull request refactors the Alloy stack setup and configuration to simplify secret management for remote endpoints. The main change is that all remote endpoint passwords (for Prometheus and Loki) are now expected in a single Kubernetes secret (grafana-alloy-secrets) using a conventional key naming scheme. This removes the need for ExternalSecret and ClusterSecretStore configuration, clarifies local development workflows, and updates documentation and code to reflect the new convention.

Secret management and configuration simplification:

• Removed support for ExternalSecret and ClusterSecretStore resources; all remote passwords are now sourced from the grafana-alloy-secrets K8s secret using keys like PROMETHEUS_PASSWORD_<NAME> and LOKI_PASSWORD_<NAME>. [1] [2] [3] [4]
• Added the RequiredSecrets() method to ConfigBuilder to programmatically determine required secret keys for remotes, and updated tests to validate this convention. [1] [2]

Taskfile and local development workflow improvements:

• Updated Taskfile.yaml to provide clear instructions for starting the Alloy stack, creating/deleting the K8s secret, and next steps for local testing; added new tasks for secret creation and deletion. [1] [2] [3] [4]

CLI and documentation changes:

• Updated Alloy CLI flags and help text to clarify the new secret convention, deprecated the --cluster-secret-store flag, and improved install command documentation to reflect the new workflow. [1] [2]

Code and test cleanup:

• Removed unused code and constants related to ExternalSecret, ClusterSecretStore, and Vault path prefix. [1] [2] [3] [4]
• Updated tests in internal/alloy/render_test.go to validate secret key conventions and ensure only the conventional secret is required.

Documentation updates:

• Added clarifying comments and updated documentation throughout the codebase to reflect the new secret management convention.

Related Issues

• Closes Alloy installations depending only on k8s secrets #376