REST: Standardize vended credentials used in loadTable / loadView responses

[nastra]

Proposed Change

Motivation

When a table is loaded, the OpenAPI REST spec uses a generic config field in LoadTableResult to pass a table-specific configuration that includes vended credentials, which are used to configure the respective FileIO implementation.
At this moment, only the S3-related credentials are properly documented in the OpenAPI spec as can be seen here and there have been attempts to document ADLS and GCS-related configuration options by #10576. The same config field is used in LoadViewResult but lacks proper documentation.

This proposal aims at standardizing the credential-specific configurations for S3 / GCS / ADLS in the REST spec to be used for credential vending.
This proposal is also a required predecessor for providing a mechanism to refresh vended credentials, which will be handled in a follow-up proposal.

Goals

• standardize credentials in a well-defined structure for S3 / GCS / ADLS
• allow adding credentials for other FileIO implementations in the future

Non-Goals

• defining credentials for other FileIO implementations
• standardizing other configuration options for S3 / GCS / ADLS
• refreshing vended credentials (this will be a separate proposal that depends on this proposal)

Proposal document

https://docs.google.com/document/d/1lySd_5hMZNtISLKsOvAq7xiNzdXU6TAoHF_yrOXWQvM/edit?usp=sharing

Specifications

• Table
• View
• REST
• Puffin
• Encryption
• Other

Spec changes: #10722