access/delegate should cap overall delegation size

[gobengo]

Rationale

• clients can invoke access/delegate with=space to store some ucan delegations associated with a space
• but there's currently no checking on size of those delegations
• we don't know of a use case that requires these delegations to be extremely large, so we should be able to put an upper bound on the size of these invocations without preventing important use cases
• if would be helpful to have some cap on these, at the very least for doing capacity planning (e.g. for store access-api delegation bytes in R2 not D1 #571)

What should the cap be?

• No individual delegation can be bigger than 256kB
  • after store access-api delegation bytes in R2 not D1 #571 we can increase this e.g. to 1024kb
• No access/delegate nb.delegations can be bigger than ?
  • Is this important to limit? Or should we perhaps just define an overall invocation size limit (not just for access/delegate), which I think is implicitly already limited to 100Mb due to cf worker request size limit
• we make these limits configurable and increase as needed