Add direction parameter to `/relations` api

[ismailgulek]

Description:

When we load a context around an in-thread event, we want to paginate backwards and forwards from there and load older and newer messages in a thread, respectively. We have a dir parameters in /messages api to paginate in both directions in a room timeline. We need something similar for /relations api to achieve the same thing on a thread timeline.

[clokep]

This was discussed a little bit in the MSC: matrix-org/matrix-spec-proposals#2675 (comment)

uhoreg/matrix-doc@6a76e5c might be vaguely related to this, but was essentially undone via uhoreg/matrix-doc@bcf7d15.

MSC2946 says this API is meant to act the same as /messages, maybe it was just an oversight that dir was not included?

[clokep]

matrix-org/matrix-doc@8e532ac (#2675) seems to have removed the reference to dir, maybe based on the conversation at matrix-org/matrix-spec-proposals#2675 (comment)

[clokep]

So the result of this is that this is "as-specced" from MSC2675, but I wrote MSC3715 to add them.