RFC for FaaS memory fields

[mjwolf]

Add RFC for FaaS memory fields: max_memory and memory_size. These fields will help track metrics and logs related to FaaS memory usage.

• Have you signed the contributor license agreement? y
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• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? y
• If submitting code/script changes, have you verified all tests pass locally using make test? y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? y
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. y
• Have you added an entry to the CHANGELOG.next.md? n/a

[mjwolf]

Open Telemetry Semantic conventions already has faas.max_memory registry attribute and a faas.memory_usage metric