Rework leaf node extensions to work via parameters rather than as a c…

[CaioSym]

Issues:

Resolves #195

Description of changes:

• Fully remove ClientConfig.leaf_node_extensions AND ClientConfig.key_package_extensions (Breaking change.)
  • I believe breaking the existing API is necessary to ensure we avoid some of the undesirable behaviours in the discussion section
• Allow leaf_node_extensions to be passed as parameters when executing the following operations:
  • Creating a group (ie, setting the LNEs for the member that creates the group)
  • Generating a KeyPackage / KeyPackage message
  • Making Commits (via a property in CommitBuilder)
    • If the property is not passed, then the commit code should be modified to preserve whatever value is currently set in LeadNode.extensions rather than overwrite it
  • Making update proposals
    • If the property is not passed, then the commit code should be modified to preserve whatever value is currently set in LeadNode.extensions rather than overwrite it
• Allow key_package_extensions to be passed as parameters when executing the following operations:
  • Generating a KeyPackage / KeyPackage message

Call-outs:

This is a breaking change as it reworks several methods in the public API. We could mitigate some of this by introducing more methods that roll out default LNEs.

Testing:

Current unit tests already cover all of the affected areas. All tests are passing. Some tests were modified so as to leverage the new apis introduced in this PR.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[mulmarta]

Thanks @CaioSym ! Left a couple of nit comments.