feat: add meshstack api to call any meshStack API path

[JohannesRudolph]

Adds meshstack api <path>, in the style of gh api: it sends an authorized request to a path of the endpoint and writes the answer unparsed.

This covers what no command wraps yet, without one command per kind: deleting or purging building blocks, tenants and projects, and reading v2-preview media types (lifecycle timestamps and authors).

• -X method, -H headers (Accept picks the API version), --input file or - for stdin.
• A non-2xx answer still writes its body, then exits 1.
• An absolute URL is refused, so the token stays on the endpoint.
• The raw request shares the 401 retry with DoRequest. client/ is untouched.

Independent of #7, #10 and #11.

🤖 Generated with Claude Code

[meshcloud-gh-actions]

Coverage of the acceptance run against the meshStack backend, on e68a0da2e23a8b531f466359d6922bec73630451.

Scope	Coverage
Unit tests	58.1%
Acceptance tests	14.6%
Combined	58.4%

Uncovered functions

client/api_key.go:48: meshApiKeyClient.Create 0.0%
client/api_key.go:52: meshApiKeyClient.Read 0.0%
client/api_key.go:56: meshApiKeyClient.Update 0.0%
client/api_key.go:60: meshApiKeyClient.Delete 0.0%
client/api_key_permissions.go:21: ApiKeyPermissions.AllCodes 0.0%
client/api_key_permissions.go:34: ApiKeyPermissions.WorkspaceCodes 0.0%
client/api_key_permissions.go:50: ApiKeyPermissions.MarkdownString 0.0%
client/api_key_permissions.go:226: AllApiKeyPermissions 0.0%
client/api_key_permissions.go:231: WorkspacePermissionCodes 0.0%
client/building_block_definition.go:72: MeshBuildingBlockDefinitionApprovalPolicies.NothingRequiresApproval 0.0%
client/building_block_definition.go:83: DisabledSchedule 0.0%
client/building_block_definition.go:90: MeshBuildingBlockDefinitionSchedule.IsDisabled 0.0%
client/building_block_definition.go:95: MeshBuildingBlockDefinitionSpec.HasNeutralPolicies 0.0%
client/building_block_definition.go:100: MeshBuildingBlockDefinitionSpec.WithNeutralPolicies 0.0%
client/building_block_definition.go:153: meshBuildingBlockDefinitionClient.List 0.0%
client/building_block_definition.go:160: meshBuildingBlockDefinitionClient.Read 0.0%
client/building_block_definition.go:164: meshBuildingBlockDefinitionClient.Create 0.0%
client/building_block_definition.go:168: meshBuildingBlockDefinitionClient.Update 0.0%
client/building_block_definition.go:172: meshBuildingBlockDefinitionClient.Delete 0.0%
client/building_block_definition_version.go:100: TagInputTargetsFor 0.0%
client/building_block_definition_version.go:245: meshBuildingBlockDefinitionVersionClient.List 0.0%
client/building_block_definition_version.go:251: meshBuildingBlockDefinitionVersionClient.Create 0.0%
client/building_block_definition_version.go:260: meshBuildingBlockDefinitionVersionClient.Update 0.0%
client/building_block_definition_version_implementation.go:78: MeshBuildingBlockDefinitionImplementation.InferTypeFromNonNilField 0.0%
client/building_block_definition_version_implementation.go:99: MeshBuildingBlockDefinitionImplementation.MarshalJSON 0.0%
... and 169 more

covdata func names a method without its receiver, so an entry can belong to an implementation nothing selects rather than to a function the tests never reached. Open the file and line before reading one as a coverage gap.