Add end-to-end encryption when creating session.

[SecondeJK]

This PR adds the end to end encryption flag when starting an OpenTok Session.

Description

See the docs for more context:
https://tokbox.com/developer/guides/end-to-end-encryption/

Motivation and Context

New features being rolled into our SDKs

How Has This Been Tested?

Added new test and modified existing one to make sure defaults are read properly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #327 (1427f7f) into main (c85f458) will increase coverage by 2.0%.
The diff coverage is 100.0%.

❗ Current head 1427f7f differs from pull request most recent head 811cc98. Consider uploading reports for the commit 811cc98 to get more accurate results

Impacted Files	Coverage Δ
src/OpenTok/OpenTok.php	91.4% <100.0%> (+0.3%)	⬆️
src/OpenTok/Session.php	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

[jeffswartz]

Should we rev the version (in Client.php and elsewhere) to 4.12.0?

[SecondeJK]

@jeffswartz I've not bumped the hardcoded version number as #328 will rewrite how this is handled (actually addresses a TODO from 2014 on dynamic user agent string handling)

[jeffswartz]

Please add a commit to rev the minor version.