Remove `@opentelemetry/core` RPC metadata APIs

[nicohrubec]

Vendor the RPC metadata context helpers currently imported from @opentelemetry/core.

Used in packages/node-core/src/integrations/http/httpServerSpansIntegration.ts (getRPCMetadata is not used in current source but belongs to the same context-key module).

• getRPCMetadata
• setRPCMetadata
• RPCType
• RPCMetadata

[linear-code]

JS-3047