Add Membership operations to the Museum example

[ricklarsen]

Museums often offer annual memberships to guest with special perks. This could be implemented into the spec.

Some methods could include:

Add member Update member Change membership upgrade membership downgrade membership cancel membership Find member Apply member discount

add membership level update membership level

[ricklarsen]

first attempt: https://github.com/Redocly/museum-openapi-example/pull/20