Make the JWS/JWE builders truly immutable and deprecate create()

[Spomky]

Description

The JWSBuilder and JWEBuilder mix three responsibilities: they are shared services holding an
AlgorithmManager, they accumulate mutable state, and they advertise an immutable with*() API that
they do not fully honour. The visible symptom is create(), which is not a named constructor but a
reset:

• https://github.com/web-token/jwt-framework/blob/4.2.x/src/Library/Signature/JWSBuilder.php#L59
• https://github.com/web-token/jwt-framework/blob/4.2.x/src/Library/Encryption/JWEBuilder.php#L68

create() only exists because the builders leak state (see the 4.2.x bug reports linked below), and
NestedTokenBuilder has to call it defensively before every build.

Three consequences, all of them user-visible:

1. State leaks — JWSBuilder::addSignature() mutates the receiver, so a shared service can be
   poisoned for the rest of the process.
2. Order-sensitive API — addRecipient() must be called after the shared protected header
   (otherwise Parameter "enc" is missing.), and calling a shared-header setter after a recipient
   raises a fatal error. Both come from validating during accumulation instead of at build().
3. Untyped internal state — $signatures and $recipients are arrays of arrays, which is how the
   dead sender_key entry survived unnoticed.

Example

Target design for 5.0.0 — the service is stateless, the state is an immutable value:

// before (4.x)
$jws = $jwsBuilder
    ->create()                       // reset, mandatory in some flows, pointless in others
    ->withPayload($payload)
    ->addSignature($key, $header)
    ->build();

// after (5.0.0)
$jws = JWSBuilder::create($algorithmManager)   // real named constructor
    ->withPayload($payload)
    ->addSignature($key, $header)              // pure clone, no cross-validation here
    ->build();                                 // every check happens here, order no longer matters

Plan for 4.3.0 (no BC break)

• Make every with*() / add*() method purely functional (clone first, never touch $this).
• Move all cross-field validation from the accumulation methods to build(), so that any call order
  works. Keep the current exception types and messages.
• Replace the internal array{key: JWK, header: array, ...} entries by @internal readonly value
  objects (SignatureSpec, RecipientSpec).
• Deprecate JWSBuilder::create() and JWEBuilder::create() with trigger_deprecation(): they become
  no-ops once the builders are immutable.
• Add a static create(AlgorithmManager $algorithms): static named constructor as the replacement.

Plan for 5.0.0

• Remove the create() reset; keep only the static named constructor.
• Make the builders final (depends on the service-interfaces issue) and the state properties
  readonly.

Related

• 4.2.x bugs JWEBuilder: fatal error when a shared header is set after a recipient has been added #678 (shared header after addRecipient()), JWSBuilder::addSignature() mutates the builder instance instead of the clone #679 (addSignature() mutating the receiver)
  and JWEBuilder::withSenderKey() cannot be used with direct key agreement (ECDH-SS) #680 (unusable withSenderKey()) — all three are symptoms of this design.