Added support for endpoints in httpcheckreceiver

[VenuEmmadi]

Description

This PR enhances the httpcheckreceiver by adding support for multiple endpoints (endpoints). Users can now specify a list of endpoints in addition to a single endpoint for each target. This improves flexibility and reduces redundancy when monitoring multiple similar endpoints.

Additional changes include:

• Updates to config.go to handle endpoints.
• Updates to scraper.go to iterate over and scrape all specified endpoints.
• Added unit tests for the new functionality in config_test.go and scraper_test.go.
• Updated documentation (README.md) to reflect the changes.

Link to Tracking Issue

Fixes #37121

Testing

• All existing and new tests pass.
• Tested the httpcheckreceiver manually using the following configuration:

receivers:
  httpcheck:
    collection_interval: 30s
    targets:
      - method: "GET"
        endpoints:
          - "https://opentelemetry.io"
      - method: "GET"
        endpoints: 
          - "http://localhost:8080/hello"
          - "http://localhost:8080/hello"
        headers:
          Authorization: "Bearer eyJhbGciOiJIUzUxMiJ9.eyJzdWIiOiJqYXZhaW51c2UiLCJleHAiOjE3MzcwMzMzMTcsImlhdCI6MTczNzAxNTMxN30.qNb_hckvlqfWmnnaw2xP9ie2AKGO6ljzGxcMotoFZg3CwcYSTGu7VE6ERsvX_nHlcZOYZHgPc7_9WSBlCZ9M_w"   
      - method: "GET"
        endpoint: "http://localhost:8080/hello"
        headers:
          Authorization: "Bearer eyJhbGciOiJIUzUxMiJ9.eyJzdWIiOiJqYXZhaW51c2UiLCJleHAiOjE3MzcwMzMzMTcsImlhdCI6MTczNzAxNTMxN30.qNb_hckvlqfWmnnaw2xP9ie2AKGO6ljzGxcMotoFZg3CwcYSTGu7VE6ERsvX_nHlcZOYZHgPc7_9WSBlCZ9M_w"
processors:
  batch:
    send_batch_max_size: 1000
    send_batch_size: 100
    timeout: 10s
exporters:
  debug:
    verbosity: detailed
service:
  pipelines:
    metrics:
      receivers: [httpcheck]
      processors: [batch]
      exporters: [debug]

Documentation

Describe any documentation changes or additions:

<!-- Describe the documentation added. -->
#### Documentation
- Updated the `README.md` to include examples for `endpoints`.
- Verified `documentation.md` for metric output consistency.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: VenuEmmadi (0b98a2d, e79a902, aec478e, ea81c9e, adb84f7, 07d4b2a, 09d4c88, e7d8ebd, f7b9cdc, ad8b022)

[VenuEmmadi]

Hi @codeboten and @mx-psi , I hope you're doing well. When you have some time, could you kindly review and share your feedback on this PR? Your input would be greatly appreciated. Thank you!