Add binary content extraction

[navarone-feekery]

Add binary content extraction support.
When a binary content file (such as a PDF) is encountered, Crawler will download the file and create an ES doc for ingestion.
The binary content of the file is added as a base64-encoded string to the _attachment field. The actual decoding is done through ES pipelines, which takes the decoded value and adds it to the body field on the doc. The _attachment field is then removed.

This can be enabled or disabled through the following configuration fields

# This example config will allow PDFs and MS word docs to be extracted

content_extraction_enabled: true
content_extraction_mime_types:
  - application/pdf
  - application/msword

This feature additionally closes #71

Checklists

Pre-Review Checklist

• This PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check crawler.yml.example and elasticsearch.yml.example)
• This PR has a meaningful title
• This PR links to all relevant GitHub issues that it fixes or partially addresses
  • If there is no GitHub issue, please create it. Each PR should have a link to an issue
• this PR has a thorough description
• Covered the changes with automated tests
• Tested the changes locally
• Added a label for each target release version (example: v0.1.0)
• Considered corresponding documentation changes
• Ran make notice if any dependencies have been added

Release Note

Crawler can now extract binary content from files it encounters during a crawl.

[seanstory]

looking great. Good tests as always. :)