[security] transport.Controller{} and transport.Transport{} security and performance improvements (#564)

* cache transports in controller by privkey-generated pubkey, add retry logic to transport requests

Signed-off-by: kim <grufwub@gmail.com>

* update code comments, defer mutex unlocks

Signed-off-by: kim <grufwub@gmail.com>

* add count to 'performing request' log message

Signed-off-by: kim <grufwub@gmail.com>

* reduce repeated conversions of same url.URL object

Signed-off-by: kim <grufwub@gmail.com>

* move worker.Worker to concurrency subpackage, add WorkQueue type, limit transport http client use by WorkQueue

Signed-off-by: kim <grufwub@gmail.com>

* fix security advisories regarding max outgoing conns, max rsp body size

- implemented by a new httpclient.Client{} that wraps an underlying
  client with a queue to limit connections, and limit reader wrapping
  a response body with a configured maximum size
- update pub.HttpClient args passed around to be this new httpclient.Client{}

Signed-off-by: kim <grufwub@gmail.com>

* add httpclient tests, move ip validation to separate package + change mechanism

Signed-off-by: kim <grufwub@gmail.com>

* fix merge conflicts

Signed-off-by: kim <grufwub@gmail.com>

* use singular mutex in transport rather than separate signer mus

Signed-off-by: kim <grufwub@gmail.com>

* improved useragent string

Signed-off-by: kim <grufwub@gmail.com>

* add note regarding missing test

Signed-off-by: kim <grufwub@gmail.com>

* remove useragent field from transport (instead store in controller)

Signed-off-by: kim <grufwub@gmail.com>

* shutup linter

Signed-off-by: kim <grufwub@gmail.com>

* reset other signing headers on each loop iteration

Signed-off-by: kim <grufwub@gmail.com>

* respect request ctx during retry-backoff sleep period

Signed-off-by: kim <grufwub@gmail.com>

* use external pkg with docs explaining performance "hack"

Signed-off-by: kim <grufwub@gmail.com>

* use http package constants instead of string method literals

Signed-off-by: kim <grufwub@gmail.com>

* add license file headers

Signed-off-by: kim <grufwub@gmail.com>

* update code comment to match new func names

Signed-off-by: kim <grufwub@gmail.com>

* updates to user-agent string

Signed-off-by: kim <grufwub@gmail.com>

* update signed testrig models to fit with new transport logic (instead uses separate signer now)

Signed-off-by: kim <grufwub@gmail.com>

* fuck you linter

Signed-off-by: kim <grufwub@gmail.com>

internal/processing/account/account.go

@@ -23,6 +23,7 @@ import (
 	"mime/multipart"
 
 	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/concurrency"
 	"github.com/superseriousbusiness/gotosocial/internal/db"
 	"github.com/superseriousbusiness/gotosocial/internal/federation"
 	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
@@ -33,7 +34,6 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/text"
 	"github.com/superseriousbusiness/gotosocial/internal/typeutils"
 	"github.com/superseriousbusiness/gotosocial/internal/visibility"
-	"github.com/superseriousbusiness/gotosocial/internal/worker"
 	"github.com/superseriousbusiness/oauth2/v4"
 )
 
@@ -84,7 +84,7 @@ type Processor interface {
 type processor struct {
 	tc           typeutils.TypeConverter
 	mediaManager media.Manager
-	clientWorker *worker.Worker[messages.FromClientAPI]
+	clientWorker *concurrency.WorkerPool[messages.FromClientAPI]
 	oauthServer  oauth.Server
 	filter       visibility.Filter
 	formatter    text.Formatter
@@ -94,7 +94,7 @@ type processor struct {
 }
 
 // New returns a new account processor.
-func New(db db.DB, tc typeutils.TypeConverter, mediaManager media.Manager, oauthServer oauth.Server, clientWorker *worker.Worker[messages.FromClientAPI], federator federation.Federator, parseMention gtsmodel.ParseMentionFunc) Processor {
+func New(db db.DB, tc typeutils.TypeConverter, mediaManager media.Manager, oauthServer oauth.Server, clientWorker *concurrency.WorkerPool[messages.FromClientAPI], federator federation.Federator, parseMention gtsmodel.ParseMentionFunc) Processor {
 	return &processor{
 		tc:           tc,
 		mediaManager: mediaManager,