[dotnet] Remove Microsoft.IdentityModel.Tokens as dependency

[nvborisenko]

Description

We can remove Microsoft.IdentityModel.Tokens dependency.

What is done:

• Adopted small piece code what we need
• Remove everything related to Microsoft.IdentityModel.Tokens

Original source: https://raw.githubusercontent.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/6.19.0/src/Microsoft.IdentityModel.Tokens/Base64UrlEncoder.cs

Motivation and Context

Ideally any library should not have dependencies.
Related to #12692

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

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[titusfortner]

This looks good to me, will be nice to remove the dependency.