Potential exposure to CVE-2021-3918 - Score 9.8

Nobody has claimed this yet.

## Steps to reproduce

1. Install loopback-connector-rest
2. run `npm ls json-schema`

## Current Behavior

The vulnerable version of json-schema is a sub-dependency of `request@2.88.2` which is the latest version of the deprecated http client.
```
└─┬ loopback-connector-rest@4.0.1
└─┬ request@2.88.2
└─┬ http-signature@1.2.0
└─┬ jsprim@1.4.1
└── json-schema@0.2.3
```

## Expected Behavior

Usage of non-deprectated package which are not exposed to security vulnerabilities.

## Additional information
https://nvd.nist.gov/vuln/detail/CVE-2021-3918
Fixes exist for `json-schema`, `jsprim` and `http-signature` but `request` does not accept `http-signature@1.3.6` which would resolve this issue:
https://github.com/joyent/node-http-signature/blob/master/CHANGES.md#136

## Related Issues
https://github.com/loopbackio/loopback-connector-rest/issues/147

Start with the loopback-connector-rest dependency chain shown by `npm ls json-schema`, especially `request@2.88.2`, `http-signature`, and `jsprim`. Review the related issue and the linked change history for `http-signature@1.3.6`; done means the connector no longer brings in the vulnerable `json-schema` version without introducing a deprecated dependency.

Written by the indexing model from the issue text.