Feat/graph proxy endpoint

[vimal-vijayan]

Description

This PR adds support for overriding Microsoft Graph settings in the AzureAD provider to support proxy/custom Graph deployments.

Changes included:

• Adds new provider arguments:
  • microsoft_graph_endpoint (env: ARM_MS_GRAPH_ENDPOINT)
  • microsoft_graph_resource_id (env: ARM_MS_GRAPH_RESOURCE_ID)
• Trims trailing / from microsoft_graph_endpoint for consistent behavior.
• Preserves the Microsoft Graph App ID when endpoint/resource overrides are applied.
• Adds validation and existing safety checks to ensure Graph endpoint resolution remains valid.
• Updates provider docs for the new arguments.
• Adds acceptance tests for:
  • endpoint-only override
  • endpoint + resource ID override

Changes to existing Resource / Data Source

• I have added an explanation of what my changes do and why I'd like you to include them.
• I have written new tests for my changes & updated relevant documentation.
• I have successfully run tests with my changes locally. (update this checkbox based on your run)
• (For changes that include a state migration only). I have manually tested the migration path between relevant versions of the provider.

Testing

• Added:
  • TestAccProvider_graphEndpointOverride
  • TestAccProvider_graphEndpointAndResourceIdOverride

Example local run command:

TF_ACC=1 go test ./internal/provider -run 'TestAccProvider_graphEndpointOverride|TestAccProvider_graphEndpointAndResourceIdOverride'