Synapse allows invalid characters in the signature key id

[matrixbot]

This issue has been migrated from #8307.

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Synapse seems to accept more characters in the signature key id than the spec allows (for example +).

In Matrix HQ there's an event with signatures like this:

        "signatures": {
            "solver.nu": {
                "ed25519:i+b2": "eodPQHXrns8Jk0XITTlaB61XdjxJW8uCi7paKqgrJmA5ok0NfsRw4Zhyx9RaIs/e7tZMJ29O46oh0IRx6jwZCQ"
            }
        }

But the spec says only [a-zA-Z0-9_] is allowed (https://matrix.org/docs/spec/server_server/r0.1.4#post-matrix-key-v2-query)