fix(nested-clients): create nested clients for internal use

[kuhe]

Issue

#6258
#5818 etc.

• multiple tickets related to either circular dependencies, incorrect peers, or bundle size

Description

• create nested STS and SSO-OIDC clients in a separate package
• compress rulesets
• generate with pared down operation list
• investigate what other optional components can be removed

Testing

• CI passing with existing credential provider tests

[kuhe]

• create submodules using package.json imports similar to what was done in @aws-sdk/core.