fix(client-sts): allow overwriting default role assumer http handler

[AllanZhengYP]

Issue

Resolves #2356

Description

This change allows users to overwrite the requestHandler of the STS client used in the default role assumers from
getDefaultRoleAssumer and getDefaultRoleAssumerWithWebIdentity. As a result, you can specify the http options
of the under lying STS client like socketTimeout, connectionTimeout and the httpsAgent etc.

Testing

unit test

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