Fix type disagreement for required_headers in service code

[daneah]

This change is a: (check at least one)

• Bugfix
• Feature addition
• Code style update
• Refactor
• Release activity

Is this a breaking change? (check one)

• Yes
• No

Is the:

• Title of this pull request clear, concise, and indicative of the issue number it addresses, if any?
• Test suite passing?
• Code coverage maximal?
• Changelog up to date?

What does this change address?

Consumers of apiron could run into type errors when overriding required_headers in a service they define.

• The required_headers attribute in ServiceBase is writeable, but consuming code that tried to override required_headers as a property would make required_headers read-only, which breaks the Liskov substitutability principle.
• The type of the required_headers attribute in ServiceBase was not as restrictive as the return type of the required_headers property in ServiceMeta, potentially leading to type disagreements in consuming code

How does this change work?

• Update required_headers in the ServiceBase class to be a property, matching the definition in ServiceMeta and making it overridable as a property in consuming code.
• Ensure the return type is as restrictive in ServiceBase as in ServiceMeta.

[NazimHAli]

🎉