Split rewrite and do some refactoring so that we have RewritePlan and RewriteExecution

[jaggederest]

This is a speculative refactor so don't burn too many cycles, just mostly interested in strategic feedback.

[codecov]

Codecov Report

❌ Patch coverage is 32.97872% with 63 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/rewrite/mod.rs	37.20%	27 Missing ⚠️
pgdog/src/frontend/client/query_engine/mod.rs	21.05%	15 Missing ⚠️
pgdog/src/frontend/client/query_engine/rewrite.rs	0.00%	11 Missing ⚠️
pgdog/src/frontend/router/parser/query/mod.rs	55.00%	9 Missing ⚠️
pgdog/src/frontend/router/parser/command.rs	0.00%	1 Missing ⚠️

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