v2 of XLS-68: Sponsored Fees and Reserves specification

[mvadari]

Changelog:

• New object, Sponsorship, that can be used to track fee and reserve allocations from a sponsor to a sponsee
• The signature in the transaction is now optional if the Sponsorship object exists - the sponsor can specify if they want to require signatures for each transaction or not (this allows the sponsoring fee flow to be much smoother)
• Generally more fleshed-out details everywhere in the spec