[communication-identity] Add methods to translate CommunicationIdentifier to / from a stable string format

[prprabhu-ms]

Description

The CommunicationIdentifier design provides a good abstraction of Azure Communication Services internal id format with better type safety, auto-complete and hides internal knowledge.

However, it doesn't lend well to storing identifiers in a database or using them as keys because there is no clear canonical way how to encode them. This PR introduces translation functions that lets developers use the underlying raw ID for these purposes.

Details Internal wiki

This PR closely follows the already merged JS PR, but diverges in two points:

• The types already expose a raw_id property. This PR just makes sure that property is populated on construction of the object.
• There is no equivalent of JS' CommunicationIdentifierKind structured type in python.

No REST changes.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-communication-identity

[petrsvihlik]

👍

[DominikMe]

Lgtm! Thanks a lot for implementing this in Python.