chore(token-providers): add feature ID 'BEARER_SERVICE_ENV_VARS'

[trivikr]

Issue

Internal JS-5860

Description

Adds feature ID 'BEARER_SERVICE_ENV_VARS'

Testing

Unit tests in CI

In internal test setup, the feature ID 3 is populated with token is set

$ NODE_DEBUG=http node test.mjs 2>&1 | grep "'user-agent'"
    'user-agent': 'aws-sdk-js/3.840.0 ua/2.1 os/darwin#24.5.0 lang/js md/nodejs#22.16.0 api/bedrock#3.840.0 m/N,E,3',

The feature ID 3 is not populated when token is not set. It instead populates feature IDs w and v for credentials.

$ NODE_DEBUG=http node test.mjs 2>&1 | grep "'user-agent'"
    'user-agent': 'aws-sdk-js/3.840.0 ua/2.1 os/darwin#24.5.0 lang/js md/nodejs#22.16.0 api/bedrock#3.840.0 m/N,E,w,v',

Additional context

PR to set credential attribution for reference #6546

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