Add oxide_external_subnets data source

[sudomateo]

Summary

• add the oxide_external_subnets collection data source for SSE-285
• accept a project name or ID and list all project external subnets across API pages
• expose external subnet metadata, allocation IDs, CIDR, attachment, and timestamps
• add an acceptance test, example configuration, and generated documentation

Tests

• make test (222 tests; 146 acceptance-gated skips)
• make lint
• make testacc-local TEST_ACC=github.com/oxidecomputer/terraform-provider-oxide/internal/provider/external_subnets TEST_ACC_NAME=TestAccDataSourceExternalSubnets_full