Implement backup command (serverless index backups / restore jobs) (#62)

## Problem
Backups and restore jobs for serverless indexes have been supported in
the `go-pinecone` SDK for a while, but they haven't been brought to the
CLI.

## Solution
- Implement backup command: create, list, describe, delete. 
- Implement backup restore command: list, describe - backup restore
supports restoring indexes from backups as well. - Add unit tests for
`backup` and `restore` commands.
- Update `--json` flags to support shorthand `-j` across all commands.

## Type of Change
- [ ] Bug fix (non-breaking change which fixes an issue)
- [X] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update
- [ ] Infrastructure change (CI configs, etc)
- [ ] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

## Test Plan
CI - unit & integration tests

There were new unit tests added for the backup commands. I've also
tested the flow of index -> backup -> restore job -> index manually.

```bash
pc backup create --index-name my-index --name my-index-backup --description "backup of my-index"

# list all backups for a project
pc backup list

# list backups for a specific index
pc backup list --index-name my-index

# describe backup and restore an index from that backup
pc backup describe --id backup-id-123-456
pc backup restore --id backup-id-123-456 --name my-index-restored

# list restore jobs
pc backup restore list
```


---
- To see the specific tasks where the Asana app for GitHub is being
used, see below:
  - https://app.asana.com/0/0/1210811533821769

Author: austin-denoble

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