storcon: reuse reqwest http client

[DimasKovas]

Problem

• Part of Refactor PageserverClient and SafekeeperClient #11113
• Building a new reqwest::Client for every request is expensive because it parses CA certs under the hood. It's noticeable in storcon's flamegraph.

Summary of changes

• Reuse one reqwest::Client for all API calls to avoid parsing CA certificates every time.

[github-actions]

8019 tests run: 7636 passed, 0 failed, 383 skipped (full report)

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Flaky tests (11)

Postgres 16

• test_compute_pageserver_hung_connections: release-arm64-with-lfc
• test_sliding_working_set_approximation: release-arm64-with-lfc

Postgres 15

• test_compute_pageserver_statement_timeout: release-arm64-with-lfc
• test_lfc_working_set_approximation: release-arm64-with-lfc
• test_pitr_gc: release-x86-64-with-lfc
• test_scrubber_tenant_snapshot[4]: release-arm64-with-lfc
• test_safekeeper_without_pageserver: release-x86-64-with-lfc

Postgres 14

• test_compute_pageserver_hung_connections: release-x86-64-with-lfc
• test_sliding_working_set_approximation: release-arm64-with-lfc
• test_lfc_working_set_approximation: release-arm64-with-lfc
• test_get_tenant_size_with_multiple_branches: release-x86-64-with-lfc

Code coverage* (full report)

• functions: 32.3% (8735 of 27049 functions)
• lines: 48.4% (75327 of 155531 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
3e401e2 at 2025-03-21T11:12:50.500Z :recycle:}