fix(#435): use TryAddSingleton when registering IStorageInitializer to avoid overwriting user configured initialisers

[esskar]

Description:

It was not obvious how this feature works. adding a custom IStorageInitializer before calling UseCap is not working as the registration will get overwritten. This is not the case for the MySQL Initializer as this was already using the TryAddSingleton functionality

Issue(s) addressed:

• Support custom table names #435

Changes:

• Calling TryAddSingleton when registering IStorageInitializer

Affected components:

• InMemoryCapOptionsExtensions
• MongoDBCapOptionsExtensions
• PostgreSqlCapOptionsExtensions
• SqlServerCapOptionsExtensions

Additional notes (optional):

I noticed that setting a custom IStorageInitializer

Checklist:

• I have tested my changes locally
• I have added necessary documentation (if applicable)
• I have updated the relevant tests (if applicable)
• My changes follow the project's code style guidelines