increase custom header len to 6000 and migrated db

[tiagoshin]

Description

New schema migration for the field custom_header in search_endpoints database.

Motivation and Context

We had custom headers that were longer than the previously set.

How Has This Been Tested?

Manually

Screenshots or GIFs (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [] Improvement (non-breaking change which improves existing functionality)
• [] New feature (non-breaking change which adds new functionality)
• [] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• [] My change requires a change to the documentation.
• [] I have updated the documentation accordingly.
• [] I have read the CONTRIBUTING document.
• [] I have added tests to cover my changes.
• [] All new and existing tests passed.