Add lock to websocket Connections method

[lorenzodonini]

Proposed changes

Fixes the new Connections method introduced in #284 by adding a RLock/unlock call to make the function thread-safe.

See https://github.com/lorenzodonini/ocpp-go/pull/284/files#r1921637697

Types of changes

What types of changes does your code introduce?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)