Etcd/move schemas to file

[etcd]

Moves the schemas (utility types) into a separate file so zod schemas for runtime type-checking can be added without cluttering up the client code.

(This is a draft PR, stacked on top of a parent that currently needs to be merged first! The diff will be smaller once the parent is merged. The GitHub UI doesn't seem to let me change the diffbase, as the parent branch lives on my fork. Before merging, first merge the parent and rebase this PR on main.)

Parent:

• https://github.com/turbopuffer/turbopuffer-typescript/pull/9

Child:

• https://github.com/turbopuffer/turbopuffer-typescript/pull/11

The whole stack (in order):

• https://github.com/turbopuffer/turbopuffer-typescript/pull/8
• https://github.com/turbopuffer/turbopuffer-typescript/pull/9
• https://github.com/turbopuffer/turbopuffer-typescript/pull/10 (this)
• https://github.com/turbopuffer/turbopuffer-typescript/pull/11

[etcd]

Gonna close this one due to drift from main - the changes are easy enough to redo from scratch off of main and put in a new branch.