Extend date parsing implementation

As a follow up to https://github.com/swift-server/swift-aws-lambda-events/pull/81

> Given this is being used with more than HTTP headers I think we need to extend the number of timezone identifiers. The list is missing standard ones listed in RFC PST, EST, MST, CST (funnily RFC only lists US zones). Also does the parser deal with additional characters at the en. It is common to see email dates written as follows Sun, 22 Sep 2024 18:46:31 +0100 (BST) with the three character timezone at the end.

Start with the date parsing implementation discussed in pull request #81, then inspect how timezone identifiers and trailing characters are handled. Confirm the expected standard identifiers, including PST, EST, MST, and CST, and test the example date with the trailing (BST); done means these inputs parse correctly.

Written by the indexing model from the issue text.