Add account_id to credentials object

[alexgromero]

Overview

This PR introduces a new optional field, account_id, to the Credentials object. The account_id will default to None if not supplied. This addition enables the retrieval of an AWS account ID during credential resolution, which will then be used in the endpoint resolution process to construct endpoints that require an account ID.
The field will be added to the following classes:

• Credentials
• RefreshableCredentials
• DeferredRefreshableCredentials
• ReadOnlyCredentials

Testing

Testing for this change will be included in upcoming related PRs.

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Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (account-id-endpoint-routing@f1b5714). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
botocore/credentials.py	83.33%	2 Missing ⚠️

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Additional details and impacted files

@@                      Coverage Diff                       @@
##             account-id-endpoint-routing    #3328   +/-   ##
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