Add path `Prefix` option

[jamietanna]

oapi-codegen/echo-middleware#15 / #40

[ChepChaf]

I have an use case where we use /api/v{n} as a basepath for the API and the middleware isn't able to handle it, this PR would fix it, any way I can help?

[jamietanna]

Hey, that's awesome! I guess any additional test cases we can put into #40 - and once we get round to finishing off that PR doing a test with your own workload, that'd be awesome 🤞🏽