Add custom header for Avon Transit

[ianktc]

Summary:

Add custom User-Agent header=MobilityDatabaseAgent for Avon Transit. I verified that without any header, the response using python request lib is 404 not found. When using our new custom user agent (MobilityDatabaseAgent) we're able to get a 200 response.

MobilityData/mobility-database-catalogs#1704

Liquibase Changes:

• Added a new migration, add_avon_transit_header.sql, to insert a custom User-Agent header override for Avon Transit (mdb-2433): (liquibase/changes/add_avon_transit_header.sql)
• Updated the main Liquibase changelog (changelog.xml) to include the new hotfix migration, ensuring it is applied in the correct order. (liquibase/changelog.xml)

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• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
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