proxy: Read cplane JWT with clap arg

[cloneable]

This is mostly a no-op. It just moves where we read the env var from somewhere deep in the code to init code. We do introduce a new flag, which might come in handy.

[github-actions]

5328 tests run: 5106 passed, 0 failed, 222 skipped (full report)

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Flaky tests (1)

Postgres 17

• test_readonly_node_gc: debug-x86-64

Code coverage* (full report)

• functions: 31.6% (7798 of 24711 functions)
• lines: 49.0% (61163 of 124797 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c6b54a7 at 2024-11-06T10:20:59.636Z :recycle:}