Install an extension

## Usage

`circleci extension install <extension> [flags]`

## Flags

Global flags: `-c, --config`, `--debug`, `--no-color`, `-q, --quiet` — see `circleci --help`.

## Examples

- Install the testsuite extension: 
  `circleci extension install testsuite`

## Details

Install a CircleCI CLI extension from the extension registry.

The extension binary is downloaded, its SHA-256 checksum is verified
against the release manifest, and the binary is written to the
extension directory.

A manifest file is written alongside the binary recording the version,
checksum, and source URL so the extension can be upgraded or removed later.

