Change how request modifiers behave for slices and maps.

[averche]

Description

For any slice value (request/response callbacks + MFA credentials), the request-level modifiers will be appended to the global client-level ones.

For any map value (custom headers), the request-level modifies will be be appended overwriting the existing keys (maps.Copy semantics).

This seems to be a more intuitive approach but I'm open to suggestions.

Example

For the code below:

client.SetRequestCallbacks(func(req *http.Request) {
	log.Println("client level")
})

resp, err := client.Secrets.KvV2Read(
	ctx,
	"my-secret",
	vault.WithRequestCallbacks(func(req *http.Request) {
		log.Println("request level")
	}),
)

The current output is:

request level

The out utput after this PR:

client level
request level

How has this been tested?

Added a test specific to this.