Allow Mixed User Token / Endpoint SecurityPolicies RSA / ECC

[romanett]

Proposed changes

Allow RSA Endpoints to work with ECC User Tokens.
Allow ECC Endpoints to work with RSA User Tokens.
Allow Token on None Endpoint to use ECC SecurityPolicy if no RSA Certificate is configured.

Allow client to use all Security Policies for User Tokens if a certificate is present (e.g. RSA Endpoint, but server offers only ECC User Tokens)

https://reference.opcfoundation.org/Core/Part6/v105/docs/6.8.2#Figure16

Related Issues

• Fixes UserToken encryption should not be dependent on the endpoint encryption. #2897

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.