JSON Module Support for Elixir 1.18 and CI Environment Updates

[michelboaventura]

This PR introduces conditional JSON handling and updates our CI environment:

• Adds a new PromEx.JSON module that automatically uses Elixir 1.18's built-in JSON module when available, falling back to Jason otherwise
• Makes the Jason dependency optional
• Adds Elixir 1.18 to the testing matrix
• Updates actions/cache action to v4

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• I have added unit tests to cover my changes.
• I have added documentation to cover my changes.
• My changes have passed unit tests and have been tested E2E in an example project.

[michelboaventura]

hey @akoutmos do you think this approach is a good one for we to start using JSON on the lib? Thanks!