feat: Document Custom Credential Suppliers.

[vverman]

Description

Adding documentation for Custom Credential Suppliers.

Custom Credential Suppliers enable developers to securely integrate third-party authentication directly into the Google Cloud SDKs. Custom Credential Suppliers are primarily used to handle authentication in non-standard cloud environments:

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• auth/custom_credential_supplier/aws/aws_custom_credential.go:17, tag auth_custom_credential_supplier_aws
• auth/custom_credential_supplier/okta/okta_custom_credential.go:17, tag auth_custom_credential_supplier_okta

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