Feat/add datasource to query machine types

[h3adex]

Description

This PR adds support for querying machine types using the IaaS API:
https://docs.api.stackit.cloud/documentation/iaas/version/v1#tag/Flavors/operation/v1ListMachineType

You can test it using the following data-sources:

data "stackit_machine_type" "two_vcpus_filter" {
  project_id = var.stackit_project_id
  filter     = "vcpus == 2"
}

data "stackit_machine_type" "filter_sorted_ascending_false" {
  project_id     = var.stackit_project_id
  filter         = "vcpus >= 2 && ram >= 2048"
  sort_ascending = false
}

data "stackit_machine_type" "intel_icelake_generic_filter" {
  project_id = var.stackit_project_id
  filter     = "extraSpecs.cpu == \"intel-icelake-generic\" && vcpus == 2"
}

# Returns a warning, as no matching machine type is found
data "stackit_machine_type" "no_match" {
  project_id = var.stackit_project_id
  filter     = "vcpus == 99"
}

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[Fyusel]

• Add datasource to beta

[h3adex]

E2E Tests