[feature] add worker task serialization logic (#2989)

* improved server shutdown with more precise shutdown of modules + deferring of ALL of it

* move delivery and workers into separate files

* add worker task model and Serialize() / Deserialize() methods for message types

* start adding message serialize / deserialize tests

* start adding test cases

* update body rewinding to rely on standard library mechanism of r.GetBody()

* remove request rewinding (http.Client{} should already handle this)

* standard library already handles rewinding

* improved code comment

* move the newPOST() function contents to prepare(), fits better with current API

* add Serialize() / Deserialize() implementations for Delivery{} type

* finish writing FromClientAPI sserialize / deserialize tests

* start adding FromFediAPI{} serialize / deserialize test cases

* fix FromFediAPI{} tests

* add tests for delivery message type

* fix repeat code

* missing license header

* use testrig status and accounts for marshal / unmarshaling tests

* add a specific test for checking account RSA keys are preserved

internal/httpclient/transport.go

@@ -21,7 +21,6 @@ import (
 	"net/http"
 	"time"
 
-	"codeberg.org/gruf/go-byteutil"
 	"github.com/superseriousbusiness/gotosocial/internal/gtscontext"
 )
 
@@ -45,12 +44,6 @@ func (t *signingtransport) RoundTrip(r *http.Request) (*http.Response, error) {
 		r.Header.Del("Signature")
 		r.Header.Del("Digest")
 
-		// Rewind body reader and content-length if set.
-		if rc, ok := r.Body.(*byteutil.ReadNopCloser); ok {
-			rc.Rewind() // set len AFTER rewind
-			r.ContentLength = int64(rc.Len())
-		}
-
 		// Sign the outgoing request.
 		if err := sign(r); err != nil {
 			return nil, err