Support sparse checkout

[jnilau]

Support for new YAML checkout task properties sparseCheckoutDirectories and sparseCheckoutPatterns to trigger a git sparse-checkout

If agent knob AGENT_USE_SPARSE_CHECKOUT_IN_CHECKOUT_TASK is not set, then agent will not initialize sparse-checkout regardless of whether the properties have values or not.

If agent knob AGENT_USE_SPARSE_CHECKOUT_IN_CHECKOUT_TASK is set to true and one of the properties are set, then agent will initialize the sparse-checkout process. sparseCheckoutDirectories will trigger cone mode where the checkout process will use directory-matching while sparseCheckoutPatterns will trigger non-cone mode and allow for more complicated pattern-matching. Agent will initialize cone mode and directory matching if both properties are set.

If agent knob AGENT_USE_SPARSE_CHECKOUT_IN_CHECKOUT_TASK is set to true and both properties are empty, then agent will disable the sparse-checkout process.

Issues while running the command will also fail the checkout task so that users can investigate the error.

YAML example for cone mode:

- checkout: repo
  sparseCheckoutDirectories: src

YAML example for non-code mode:

- checkout: repo
  sparseCheckoutPatterns: /* !img

[merlynomsft]

Approved with comments (see above)