feat: added sealed keyword

[opensourcevk]

1. DPoPKeyProvider.java (core/dpop/)

   • Converted DPoPKeyProvider interface to a sealed interface
   • Permits: StaticDPoPKeyProvider
   • Purpose: Restricts DPoP key provider implementations to explicitly permitted classes

2. AccessTokenStore.java (core/access_token/)

   • Converted AccessTokenStore interface to a sealed interface
   • Permits: InMemoryAccessTokenStore
   • Purpose: Restricts access token storage implementations to explicitly permitted classes

[jaaufauvre]

Thanks for the contribution.

I understand the intent of restricting implementations but converting interfaces to sealed interfaces breaks the library's public extension points (see README.md). Existing code that implements these interfaces will fail to compile after this change.

Also, please ensure all tests are passing before pushing changes:

Mockito cannot mock this class: interface com.mastercard.developer.oauth2.core.access_token.AccessTokenStore.
Sealed interfaces or abstract classes can't be mocked. Interfaces cannot be instantiated and cannot be subclassed for mocking purposes.