Spec compliance: synapse does not appear to correctly enforce strict canonicaljson in federation requests, etc

This issue has been migrated from [#13883](https://github.com/matrix-org/synapse/issues/13883).

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The [canonicaljson spec](https://spec.matrix.org/v1.3/appendices/#canonical-json) says that json requests cannot contain:

* Integers outside the range `[-2 ^ 53 + 1, 2 ^ 53 - 1]`
* Floats
* NaN, Infinity, -Infinity

... which should mean that it is impossible to sign a federation request containing such values, so all such requests should be rejected. However AFAICT Synapse does not enforce this.

Canonicaljson is used in a couple of other places too (3pid invites, E2EE, etc), and the same considerations apply there.

Start with the linked canonicaljson specification and audit Synapse's federation-request, 3pid-invite, and E2EE signing paths. Verify how each path handles out-of-range integers, floats, NaN, Infinity, and -Infinity, then confirm that every affected request is rejected consistently and add coverage for the discovered cases.

Written by the indexing model from the issue text.