Support setting cpu_request in cluster replica sizes

[alex-hunt-materialize]

Supports setting cpu_request independently of cpu_limit in cluster replica sizes.

Motivation

Resolves https://github.com/MaterializeInc/database-issues/issues/10100

More context:
https://materializeinc.slack.com/archives/C07PN7KSB0T/p1770215227882419
https://materializeinc.slack.com/archives/C0854GU86HL/p1771932457688129

Description

• Adds support for setting cpu_request independently of cpu_limit in ReplicaAllocation.

Verification

Orchestratord nightly tests should exercise this.

[github-actions]

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[kay-kim]

docs changes LGTM 😍😍 😍