Add fish shell format for export-credentials

[minjcho]

Resolves #9670

Description of changes:

• Add a new output format, --format fish, to aws configure export-credentials.
• Fish shell does not support export VAR=value; it uses set -gx VAR value.

This change lets Fish users apply exported credentials directly without extra conversion.

Usage:

aws configure export-credentials --format fish | source

Example output:

set -gx AWS_ACCESS_KEY_ID AKIAIOSFODNN7EXAMPLE
set -gx AWS_SECRET_ACCESS_KEY wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
set -gx AWS_SESSION_TOKEN token
set -gx AWS_CREDENTIAL_EXPIRATION 2023-01-01T00:00:00Z

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[Copilot]

Pull request overview

This pull request adds Fish shell support to the aws configure export-credentials command, allowing Fish shell users to export AWS credentials in a format compatible with their shell.

Changes:

• Added a new FishShellFormatter class that outputs credentials using Fish shell's set -gx syntax
• Updated the SUPPORTED_FORMATS dictionary to include the new Fish formatter
• Added comprehensive unit tests for the Fish shell formatter covering both static and temporary credentials

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
awscli/customizations/configure/exportcreds.py	Implements FishShellFormatter class and registers it in SUPPORTED_FORMATS
tests/unit/customizations/configure/test_exportcreds.py	Adds test cases for FishShellFormatter with expected output formats

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[ashovlin]

Diff looks good, thanks for opening this!

Can you run scripts/new-change to generate a changelog entry file? Set type to enhancement, category to ``configure``, and then a brief description for description.

[minjcho]

Done! Added changelog entry with scripts/new-change.