Implement RewriteTarget to allow requests to be rewritten before proxying to upstream servers

Convert RefreshSessionIfNeeded into RefreshSession

They will only be used in tests, but it doesn't play
nice with copy operations many tests use. The linter was
not happy. While the global clock needs mutexes for parallelism,
local Clocks only used it for Set/Add and didn't even use the
mutex for actual time functions.

The reflect.DeepCopy doesn't play nice with the new Lock and Clock
fields in sessions. And it added unneeded session deserialization
logic to every request.

Simplify sha256sum checking example

The previous code didn't consider other languages and hid the output of failures. For example

```
$ LANG=es_ES.UTF-8 sha256sum -c sha256sum.txt
oauth2-proxy-v7.1.3.linux-amd64/oauth2-proxy: La suma coincide
```

Move app redirection logic to its own package

Use gorilla mux for OAuth Proxy routing

* Update Alpine image version to 3.14.

* Update changelog.

Added ADFS Provider

* Update installation.md

Provide a link to the Helm chart for oauth2-proxy

* also document in latest versioned docs

* Fix - Refresh Session not working for multiple cookies (#1209)

* added comments for flattenheaders fix

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Updated Changelog.md

Co-authored-by: Rishi Kambil <rishi.kambil@lntinfotech.com>
Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Some typos

* Also corrected the typo's in the src code

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Add sensible logging flag to default setup for logger

* Add Redis lock

* Fix default value flag for sensitive logging

* Split RefreshSessionIfNeeded in two methods and use Redis lock

* Small adjustments to doc and code

* Remove sensible logging

* Fix method names in ticket.go

* Revert "Fix method names in ticket.go"

This reverts commit 408ba1a1a5c55a3cad507a0be8634af1977769cb.

* Fix methods name in ticket.go

* Remove block in Redis client get

* Increase lock time to 1 second

* Perform retries, if session store is locked

* Reverse if condition, because it should return if session does not have to be refreshed

* Update go.sum

* Update MockStore

* Return error if loading session fails

* Fix and update tests

* Change validSession to session in docs and strings

* Change validSession to session in docs and strings

* Fix docs

* Fix wrong field name

* Fix linting

* Fix imports for linting

* Revert changes except from locking functionality

* Add lock feature on session state

* Update from master

* Remove errors package, because it is not used

* Only pass context instead of request to lock

* Use lock key

* By default use NoOpLock

* Remove debug output

* Update ticket_test.go

* Map internal error to sessions error

* Add ErrLockNotObtained

* Enable lock peek for all redis clients

* Use lock key prefix consistent

* Fix imports

* Use exists method for peek lock

* Fix imports

* Fix imports

* Fix imports

* Remove own Dockerfile

* Fix imports

* Fix tests for ticket and session store

* Fix session store test

* Update pkg/apis/sessions/interfaces.go

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Do not wrap lock method

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Use errors package for lock constants

* Use better naming for initLock function

* Add comments

* Add session store lock test

* Fix tests

* Fix tests

* Fix tests

* Fix tests

* Add cookies after saving session

* Add mock lock

* Fix imports for mock_lock.go

* Store mock lock for key

* Apply elapsed time on mock lock

* Check if lock is initially applied

* Reuse existing lock

* Test all lock methods

* Update CHANGELOG.md

* Use redis client methods in redis.lock for release an refresh

* Use lock key suffix instead of prefix for lock key

* Add comments for Lock interface

* Update comment for Lock interface

* Update CHANGELOG.md

* Change LockSuffix to const

* Check lock on already loaded session

* Use global var for loadedSession in lock tests

* Use lock instance for refreshing and releasing of lock

* Update possible error type for Refresh

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

Fix URI fragment handling

Fixes #1205

Add alternative ways to generate cookie secrets to docs

Add pagewriter to upstream proxy

fix: SHOW_DEBUG_ON_ERROR environment variable not working

(Fixes #1178)

Update Changelog for release v7.1.3