[feat] - Adding StartOffsetProvider Interface and Implementing for Specific Detectors

[ahrav]

Description:

This PR introduces a new StartOffsetProvider interface to enhance the span calculation strategy for detectors where keywords are not prefixes in the credentials. The adjustableSpanCalculator has been updated to utilize this interface, allowing for more precise control over the start index in match spans. Specific detectors can optionally implement this interface to adjust the start index accordingly.

Checklist:

• [ ] Tests passing (make test-community)?
• [ ] Lint passing (make lint this requires golangci-lint)?

[rgmz]

I think every MultiPartCredentialProvider necessarily has to have a StartOffset to account for both

username = foo
password = bar

and

password = bar
username = foo