feat: Issue Credential - adds middleware

[soluchok]

Adds middleware support to the service.
Add your custom logic to the protocol by using Use(...Middleware) function.
The middleware functions will be executed every time before state execution.

This PR also includes the helper function which saves the credentials to the store.
It was done as an example of how to use the middleware function.
This function does not depend on the format of the attachment and assumes that the payload is a verifiable credential.
It can be easily changed when we have document/RFC which describes the payload for the specific format.

Note: The Use function is not exposed to the context. But, there is a possibility to use that.

service, err := ctx.Service(issuecredential.name)
if err != nil {
	/// ....
}
service.(interface {
	Use(items ...issuecredential.Middleware)
}).Use(func(next issuecredential.Handler) issuecredential.Handler {
	return issuecredential.HandlerFunc(func(metadata issuecredential.MetaData) error {
		// some logic here
		return next.Handle(metadata)
	})
})

Signed-off-by: Andrii Soluk isoluchok@gmail.com

[codecov]

Codecov Report

Merging #1859 into master will increase coverage by 0.03%.
The diff coverage is 97.18%.

@@            Coverage Diff             @@
##           master    #1859      +/-   ##
==========================================
+ Coverage   90.88%   90.91%   +0.03%     
==========================================
  Files         190      192       +2     
  Lines       14351    14377      +26     
==========================================
+ Hits        13043    13071      +28     
+ Misses        762      761       -1     
+ Partials      546      545       -1     

Impacted Files	Coverage Δ
pkg/didcomm/protocol/issuecredential/states.go	100.00% <ø> (+1.09%)	⬆️
pkg/framework/aries/default.go	82.79% <60.00%> (-1.48%)	⬇️
pkg/didcomm/protocol/issuecredential/middleware.go	100.00% <100.00%> (ø)
pkg/didcomm/protocol/issuecredential/service.go	91.33% <100.00%> (+1.05%)	⬆️
...protocol/middleware/issuecredential/middlewares.go	100.00% <100.00%> (ø)

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[llorllale]

suggest you use stack here so that the ordering is clear and that for-loop where you build the handler chain makes more sense

[soluchok]

Changed to:

func (s *Service) Use(items ...Middleware) {
	s.stack = append(s.stack, items...)
}

[llorllale]

I meant to use a stack as a data structure (hence the link to https://pkg.go.dev/github.com/golang-collections/collections/stack)....

[soluchok]

Oh, I got it :) I don't think we really need that. This is one more dependency for the project.
But I did a small refactor. Now it is understandable.

func (s *Service) Use(items ...Middleware) {
	var handler Handler = initialHandler
	for i := len(items) - 1; i >= 0; i-- {
		handler = items[i](handler)
	}

	s.middleware = handler
}

looks OK, but need to fix codecoverage