#1667 Remove cached delegation token from AuthorizeScramOverMtls

[justinweng-instaclustr]

This PR removes the field delegation_token from the struct AuthorizeScramOverMtls to avoid caching the token in AuthorizeScramOverMtls instances. Instead, we store the username used to generate delegation tokens in this struct and fetch the delegation token from the TokenTask every time it's needed.

Progress towards #1667

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #1683 will not alter performance

_{Comparing justinweng-instaclustr:1667-avoid-caching-scram_over_mtls (511ba93) with main (841ec95)}

Summary

✅ 39 untouched benchmarks