out_stackdriver: Don't overwrite ctx->client_email and ctx->private_key.

[igorpeshansky]

This fixes the use case of specifying the service_account_email and service_account_secret options explicitly.

Fixes #6969

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Testing
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• Example configuration file for the change

  [OUTPUT]
    Name        stackdriver
    Match       …
    service_account_email …
    service_account_secret …
  

• [N/A] Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

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• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

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Backporting

• Backport to latest stable release.

#8094

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[qingling128]

LGTM