Appservice API /_matrix/app/v1/transactions/ should specify maximum transaction sizes

[Half-Shot]

This means we should specify a maximum limit on the number of events sent. Unstable MSCs for Appservices such as:

• MSC3682: Sending Account Data to Application Services matrix-spec-proposals#3682
• MSC3395: Synthetic appservice events matrix-spec-proposals#3395
• MSC2409: Proposal to send typing, presence and receipts to appservices  matrix-spec-proposals#2409
• MSC3202: Encrypted appservices matrix-spec-proposals#3202

Will need to also specify their maximum sizes. This is needed because most HTTP listeners will specify a maximum body size for incoming requests, and in practice this is often exceeded when the AS receives a lot of traffic at once. For example, Nginx defaults to 1MB and while we shouldn't arbitrarily restrict the spec to what nginx supports, it's clear that an unlimited number of events will cause great pain.