[guide section]: Custom credential providers

[rcoh]

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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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