[exporter-logs-otlp-grpc] add support for logs specific env vars

[pichlermarc]

Is your feature request related to a problem? Please describe.

After #3712 merges, the following environment variables will be not yet unsupported:

• OTEL_EXPORTER_OTLP_LOGS_INSECURE
• OTEL_EXPORTER_OTLP_LOGS_CERTIFICATE
• OTEL_EXPORTER_OTLP_LOGS_CLIENT_KEY
• OTEL_EXPORTER_OTLP_LOGS_CLIENT_CERTIFICATE
• OTEL_EXPORTER_OTLP_LOGS_TIMEOUT

Describe the solution you'd like

Add support for the environment variables listed above.
We also need to add a description of the environment variables to the README.md

[pichlermarc]

Fixing #3748 is a prerequisite for this issue.

[github-actions]

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