Ensuring that cookie sessions are reusable across nodes

[pmlopes]

Motivation:

Fix #2745

This PR changes the underlying algorithm for security reasons and fixes the test that IVs are stored in the cookie so they can be decripted on other nodes, making the statement that sessions can survive a server crash correct.

Regarding the algorithm:

Feature	AES/CBC	AES/GCM
Confidentiality	Yes	Yes
Data Integrity	No (needs separate MAC)	Yes (built-in authentication tag)
Padding Requirement	Required (e.g., PKCS5Padding)	Not required
Performance	Slower (sequential encryption)	Faster (parallelizable)
IV Requirements	Unique IV required	Unique IV critical (security failure if reused)
Security Against	Vulnerable to padding oracle attacks	Resists tampering attacks
Authentication Tag	No	Yes (128-bit tag by default)
Recommended By NIST	No	Yes
Use Cases	Legacy systems, compatibility	Modern apps, TLS 1.3, cloud security

With this update we move away from AES/CBC to the recommended (by NIST) AES/GCM

[pmlopes]

@vietj please pick the right version for this issue.

• API is backwards compatible, there is a new factory method and the existing one remains but ignores the last argument.
• behavior has changed BUT the original issue was that cookies weren't interop between vert.x instances. This means that the change does not affect the previous behavior due to existing bug anyway.

If we want to be strict, then this is a 5.1.0, however, due to the security upgrade and bug fix, it may go on a 5.0.x release either.

[vietj]

I think it is fine to be in 5.0.x, please notice that now the master branch is 5.1.0-SNAPSHOT so it needs backport to the new 5.0 branch

[tsegismont]

@pmlopes it seems there are formatting issues in tests.

About the deprecated method: I assume an existing cookie couldn't be processed after the upgrade, correct? In this case, it might be worth adding a note in the breaking changes page after your merge the PR.

Thanks for this contrib!

[pmlopes]

Notes added: https://github.com/vert-x3/wiki/wiki/5.1.0-Deprecations-and-breaking-changes

[vietj]

@pmlopes I only need it for 5.0.1