Add pipeline definition variables and variable group management tools

[ryangurn]

Summary

Add 7 new tools to the existing Pipelines domain for managing pipeline definition variables and variable groups.

Tools

Develop the following tools with full parameter support, including optional ones:

pipelines_get_definition_variables: Get variables defined on a pipeline/build definition.

Endpoint: SDK getBuildApi().getDefinition() — returns definition.variables

pipelines_update_definition_variables: Update variables on a pipeline/build definition.

Endpoint: SDK getBuildApi().getDefinition() + getBuildApi().updateDefinition() — merges with existing variables

pipelines_list_variable_groups: List variable groups for a project.

Endpoint: SDK getTaskAgentApi().getVariableGroups()

pipelines_get_variable_group: Get a variable group by ID.

Endpoint: SDK getTaskAgentApi().getVariableGroup()

pipelines_create_variable_group: Create a new variable group with variables.

Endpoint: SDK getTaskAgentApi().addVariableGroup()

pipelines_update_variable_group: Update a variable group's name, description, or variables.

Endpoint: SDK getTaskAgentApi().getVariableGroup() + getTaskAgentApi().updateVariableGroup() — merges with existing variables

pipelines_delete_variable_group: Delete a variable group.

Endpoint: SDK getTaskAgentApi().deleteVariableGroup()

Rules

1. Adhere strictly to existing project standards and coding conventions.
2. Ensure each tool exposes all API parameters (required and optional).
3. Use the official Azure DevOps Node API to interact with the APIs.

[ryangurn]

I am more than willing to submit a PR for this.

I have already created this via my fork.