[v3] Implement new Concept Exercise: protocols

This issue describes how to implement the protocols concept exercise for the Swift track.

## Getting started

**Please please please read the docs before starting.** Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read the following documents:

- [The features of v3][docs-features-of-v3].
- [Rationale for v3][docs-rationale-for-v3].
- [What are concept exercises and how are they structured?][docs-concept-exercises]

Please also watch the following video:

- [The Anatomy of a Concept Exercise][anatomy-of-a-concept-exercise].

## Goal

This concept exercise should convey a basic understanding of how to work with protocols in Swift.

## Things to teach

- What protocols are
- Protocol syntax
- Protocol conformance
- Property requirements
- Method requirements
- init requirements
- Protocols as types
- Conformance and adoption via extensions

## Things not to teach

- Protocol extensions
- Delegation
- Protocol inheritance
- Protocol composition
- Protocols with associated types
- Class only protocols
- Conformance checking with `as`
- Optional requirements

## Concepts

- `protocols`

## Prerequisites

- `structs-and-classes`
- `enumerations`
- `properties`
- `methods`
- `initialization`

## Resources to refer to

### Hints

- [Protocol syntax][protocol-syntax]
- [Property requirements][protocol-property-requirements]
- [Method requirements][protocol-method-requirements]
- [Initializer requirements][protocol-initializer-requirements]
- [Protocols as types][protocol-as-type]
- [Protocols via extension][protocol-via-extension]

### After

- [Protocol syntax][protocol-syntax]
- [Property requirements][protocol-property-requirements]
- [Method requirements][protocol-method-requirements]
- [Initializer requirements][protocol-initializer-requirements]
- [Protocols as types][protocol-as-type]
- [Protocols via extension][protocol-via-extension]

## Representer

TBD

## Analyzer

TBD

## Implementing

See [the Swift track implementation guide][swift-implementing-concept-exercises] for guidance.

## Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.

[swift-implementing-concept-exercises]: https://github.com/wneumann/v3/blob/master/languages/swift/reference/implementing-a-concept-exercise.md
[protocol-syntax]: https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID268
[protocol-property-requirements]: https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID269
[protocol-method-requirements]: https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID270
[protocol-initializer-requirements]: https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID272
[protocol-as-type]: https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID275
[protocol-via-extension]: https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID277
[docs-concept-exercises]: ../../../docs/concept-exercises.md
[docs-rationale-for-v3]: ../../../docs/rationale-for-v3.md
[docs-features-of-v3]: ../../../docs/features-of-v3.md
[anatomy-of-a-concept-exercise]: https://www.youtube.com/watch?v=gkbBqd7hPrA