Add support for STS

[andyrzhao]

Added "--uat" boolean flag to perform UAT exchange, converting the fetched OAuth Access Token to UAT. This flag is designed to be compatible with oauth and sso, as well as curl and caching.

Added "--user_project" GUAC parameter for use in UAT claim.

Refactored and simplified main.go and tasks.go by introducing the concept of "taskSettings".

Refactored SSO code path to align with oauth and jwt code path, so it can take advantage of caching and uat support.

Core UAT exchange logic is in util/uat.go

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