Reject JWS tokens declaring unsupported crit header extensions

[arpitjain099]

Fixes #406

Summary

jws.verify (and therefore jwt.decode) accepts a token whose protected
header declares a critical extension that the library does not understand,
instead of rejecting it. This is a fail-open gap: a relying party that issues
or expects a crit policy marker gets no enforcement, and an attacker can
strip or ignore the semantics the issuer intended to make mandatory.

RFC 7515 4.1.11

The crit (Critical) Header Parameter "indicates that extensions to this
specification and/or [JWA] are being used that MUST be understood and
processed." The spec is explicit that:

If any of the listed extension Header Parameters are not understood and
supported by the recipient, then the JWS is invalid.

python-jose implements none of the optional crit-flagged extensions (for
example b64 from RFC 7797), so any well-formed, non-empty crit list names
an extension this library cannot honor and the token must be rejected. RFC 7515
also constrains the value itself: crit must be a non-empty array of non-empty
case-sensitive strings, so malformed values are rejected as well.

PyJWT parity

PyJWT already fails closed on the same input, raising
InvalidTokenError: Unsupported critical extension: .... After this change
python-jose matches that behavior and raises JWSError.

Change

• Add _validate_crit(header) to jose/jws.py and call it from verify
  before signature checking.
• It rejects a malformed crit (not a list, empty, or containing a non-string
  or empty entry) and any listed extension that is not in the understood set.
  The understood set is intentionally empty because the library supports none
  of the optional extensions.
• Enforcement is gated on verify=True, so it mirrors how signature checking
  is skipped when verification is disabled.

Tests

Added TestCrit in tests/test_jws.py covering:

• a correctly signed token with an unsupported crit extension is rejected
• a token without crit still verifies
• malformed crit (non-list, empty list, non-string entry) is rejected
• crit is not enforced when verify=False

The unsupported-crit and malformed-crit tests fail on the current master
and pass with this change. The full tests/test_jws.py and tests/test_jwt.py
suites pass. flake8, isort, and black are clean on the changed files.