Use PDU not *Event in HeaderedEvent

[kegsay]

Requires matrix-org/gomatrixserverlib#376

This has numerous upsides:

• Less type casting to *Event is required.
• Making Dendrite work with PDU interfaces means we can swap out Event impls more easily.
• Tests which represent weird event shapes are easier to write.

Part of a series of refactors on GMSL.

[codecov]

Codecov Report

Patch coverage: 37.27% and project coverage change: -0.20 ⚠️

Comparison is base (696cbb7) 67.35% compared to head (6d1cbb1) 67.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3073      +/-   ##
==========================================
- Coverage   67.35%   67.16%   -0.20%     
==========================================
  Files         497      497              
  Lines       53137    53140       +3     
==========================================
- Hits        35791    35690     -101     
- Misses      13772    13876     +104     
  Partials     3574     3574              

Flag	Coverage Δ
unittests	50.05% <37.27%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
appservice/consumers/roomserver.go	71.09% <0.00%> (ø)
clientapi/routing/aliases.go	69.49% <0.00%> (-2.93%)	⬇️
clientapi/routing/directory.go	60.08% <0.00%> (-0.43%)	⬇️
clientapi/routing/sendevent.go	70.85% <0.00%> (-2.70%)	⬇️
clientapi/routing/server_notices.go	70.44% <0.00%> (ø)
federationapi/internal/federationclient.go	54.07% <ø> (ø)
federationapi/routing/backfill.go	57.31% <0.00%> (-3.66%)	⬇️
federationapi/routing/events.go	42.22% <0.00%> (-4.45%)	⬇️
federationapi/routing/invite.go	43.52% <0.00%> (-0.59%)	⬇️
federationapi/routing/join.go	53.00% <0.00%> (-1.31%)	⬇️
... and 37 more

... and 10 files with indirect coverage changes

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