[guide section]: Custom credential providers

Add a guide section for implementing your own credentials providers
Things to cover:
- [ ] simple providers with `async_provide_credentials_fn`
- [ ] Using LazyCaching to wrap the provider
- [ ] More complex providers with direct implementation of `AsyncProvideCredentials`
- [ ] use with async-trait
- [ ] overall trait layout (`Credentials` implements provide credentials, blanket impls, etc.)
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Start by locating the existing guide structure and the credential-provider APIs named in the issue: async_provide_credentials_fn, LazyCaching, AsyncProvideCredentials, async-trait, and Credentials. Done means adding a guide section that explains simple and direct providers, caching, async-trait usage, and the overall trait layout.

Written by the indexing model from the issue text.