Use consistent encoding for GBEK across languages

[damccorm]

Right now, Python's GBEK and Java's GBEK expect different secret formats. This standardizes on URL-safe Base64 encoding (which Python already uses), and should allow x-lang tests to pass (this is a subset of #36418 which does work)

Part of #36214

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R: @jrmccluskey for label python.
R: @chamikaramj for label java.

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[jrmccluskey]

LGTM from the python and docstring perspective, the Java looks fine to me as well