Add customizable detail field to request table view cells

[ZsoltMolnarMBH]

Problem:
When using Wormholy in GraphQL projects, it is difficult to oversee the GraphQL operations sent, due to the endpoint being the same in every single case.

Solution:
Add a label to RequestCell that can be customized what to show.

Example usage:

Wormholy.detailProvider = { $0.headers["X-APOLLO-OPERATION-NAME"] }

Screenshot:
(Note our debug menu changes the navigation title, please ignore that).

[pmusolino]

Thank you for the changes you've made in this PR. It looks good to me and I plan to merge it soon. I think it would be helpful to include an example in the README file to show how the label customization feature can be used. Would you be able to provide a simple code snippet or usage example that we could include in the README? This would help our users understand how to use this feature more easily. Thank you!