fix: Fixes issues w/ shared pointers to structs

[harrisoncramer]

I'd introduced a middleware pattern that was validating structs. The middleware was set up in the server with pointers to structs, like this:

m.HandleFunc("/mr/merge", middleware(
    mergeRequestAccepterService{d, gitlabClient},
    withMr(d, gitlabClient),
    withPayloadValidation(methodToPayload{http.MethodPost: &AcceptMergeRequestRequest{}}), // Here...
    withMethodCheck(http.MethodPost),
))

This caused the struct data to be shared across requests! This was causing issues where payload fields were being shared between API calls. We want new payloads for every API call.

This MR adjusts the code to add a new factory function on the methodToPayload map that creates a new payload per request so that data is not shared across them.

The new pattern is like this:

m.HandleFunc("/mr/merge", middleware(
    mergeRequestAccepterService{d, gitlabClient},
    withMr(d, gitlabClient),
    withPayloadValidation(methodToPayload{http.MethodPost: newPayload[AcceptMergeRequestRequest]}),
    withMethodCheck(http.MethodPost),
))

Where newPayload creates a new instance of the type provided and returns a pointer to that value in memory.