feat: PC-12014 Support CompositesV2

[BSski]

Summary

Deprecated the composite section on top-level SLO resource and replaced it with objective level composite definition.
Besides the composite related changes, this PR makes indicator field optional, as the provider wasn't allowing applying composite SLOs which do not use it.
The changes are not yet available for our users, therefore no release note included.

[nikodemrafalski]

As per today's reminder of tech writing team, please add more context to the release notes - for example:
We deprecated the composite section on top-level SLO resource and replace it with objective level composite definition

[nikodemrafalski]

The documentation needs to be regenerated with make generate and committed. Also, just double-checking, have the documentation changes been reviewed by the tech writing team?

[annatrifonova]

Findings after the 1st round of tests:

1. Objective's display_name is required now - should be optional (this is not related to the changes made for composites, but after discussion with Bartek decided to fix it here)
2. composite block cannot be applied without objectives and components - they should be optional (same as in sloctl)
3. objectives block cannot be empty (now terraform provider is craching when it's empty) - objectives should be optional