Use symfony/clock component when available instead of non-testable time()

[Dean151]

Motivation:
Tests are already hard enough to build so that they are testing, covering, isolated and in an expected way.
Therefore, managing external dependencies is an obligation. And time is an external dependency.

With the introduction of symfony/clock in 6.3, Symfony proposes a solution for managing time in an application, with a shared instance that is overridable during the tests.

Solution:
This PR looked over the time() usage, and replaced them with SymfonyClock::now()->getTimeStamp() when available.

Workaround:
It's possible to use Symfony/Clock in your own application without this PR by using a Events::JWT_CREATED event listener to update nbf, iat and exp accordingly.

Retro-compatibility:
Since 2.x still support down to symfony 4, this dependency is optional, therefore it was only added in suggest key.
If there is a conflict with specific version of clock, we'll be able to restrict it by using the conflict key.

3.x:
This PR does not target 3.x because it's not released yet. But in a 3.x version, symfony/clock could become a requirement and allow code simplification overall.

Tests:
Current test suite still uses time(), but they could be improved to take advantage of this new testability.

[Dean151]

Sadly, this might be incompatible with:

• Replace the usage of the built-in PHP now() function with Clock #1155
• feat: 3.x migrate to "lcobucci/jwt" v5 #1157

Therefore, I do think the dependency should be flexible in the source, and we could use the one we prefer in the tests so that the end user can rely on whatever psr/clock implementation he prefers