Record the sign-in region in the cached login token

[AndrewAsseily]

Issue #, if available:

Description of changes:

• aws login caches a refresh token, and refresh tokens are regional: they have to be redeemed in the region they were issued in. Nothing currently records where the login happened, so a refresh goes to whatever region resolves when it runs. This writes that region into the cached token as signInRegion. The value is the same region used to build the signin client for the login itself, so it reflects where the sign-in actually went

• Nothing reads the field yet: the SDK side is separate. On its own this is backwards compatible, since existing readers ignore unknown fields in the token file and tokens cached before this change just won't have it.

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[ashovlin]

Should we handle the CLI's own refresh flow while we're here?

aws-cli/awscli/botocore/credentials.py

Lines 2560 to 2566 in 10b7b71

	config = botocore.config.Config(
	signature_version=botocore.UNSIGNED,
	)
	client = self._client_creator(
	'signin',
	config=config,
	)