Add bearer/JWT auth_module for /probe multi-target scraping

[sairaj225]

Problem
Some Elasticsearch/OpenSearch deployments require Authorization: Bearer <token> (OIDC/JWT-backed access tokens). Today the exporter supports userpass, apikey, aws, and tls auth_modules, but not bearer token injection for /probe.

Proposal
Add a new auth_module type: bearer (or jwt) that injects:

• Authorization: Bearer <token> header
• supports bearer.token OR bearer.token_file (mutually exclusive)
• optional options: preserved (appended as query params, same as other modules)

Example
auth_modules:
prod_bearer:
type: bearer
bearer:
token_file: /var/run/secrets/.../token
options:
pretty: "false"

Security considerations

• never log token values
• prefer token_file for k8s secret/serviceaccount mounts

Acceptance

• unit tests for token + token_file
• validation errors for misconfigurations
• README updated under “Multi-Target Scraping”