Standardize the representation of currency pairs by fdc3.instrument

Currency pairs are not currently represented in a Standard way by FDC3 Context types. An [fdc3.currency type](https://fdc3.finos.org/docs/next/context/ref/Currency) exists as does a convention for fields that represent a currency (see: https://fdc3.finos.org/docs/next/context/spec#currency-codes), but no convention or type representing a currency pair, which are commonly traded instruments.

Currencies themselves are standardized via 3 letter codes defined in [ISO 4217](https://www.iso.org/iso-4217-currency-codes.html)
Currency pairs are defined by convention based on two such code, bought currency first, sold currency second with an optional / character in between.

This proposal seeks to standardize currency pairs in the form including a / (e.g. "GBP/USD") as part of the Context Part's [field type conventions](https://fdc3.finos.org/docs/next/context/spec#other-field-type-conventions), and to standardize the key "CURRENCY_PAIR" in the `fdc3.instrument` type's `id` field.

Start with the FDC3 Context specification's field type conventions and the fdc3.instrument type, then review the linked Currency documentation and ISO 4217 reference. Determine how the proposed CURRENCY_PAIR id key and slash-separated format should be documented; done means the convention is agreed and reflected in the relevant FDC3 specification.

Written by the indexing model from the issue text.