refactor(appservice)!: Improve API and cleanup docs

[johannescpk]

Since the lib.rs was slowly growing a bit too large and covering multiple contexts it was about time to refactor a bit. Also clarified the implicit behavior which special-cases the sender_localpart user.

• Clarify why we create a virtual user for the sender_localpart on AppService construction
• Drop the methods register_event_handler and register_event_handler_context, so the consumer has to be explicit
• Drop the method get_cached_client as it's already covered by the method virtual_user
• Move create_session into the VirtualUserBuilder, as it's only used there
• Rename virtual_user_client* to virtual_user* to make it more concise
• Make NamespaceCache private since there's no reason to expose it
• Split out AppServiceRegistration and VirtualUserBuilder into their own files to make the code easier to navigate
• Clean up docs

Part of #228

[codecov]

Codecov Report

Merging #837 (ec00af0) into main (9a45325) will increase coverage by 0.04%.
The diff coverage is 60.78%.

@@            Coverage Diff             @@
##             main     #837      +/-   ##
==========================================
+ Coverage   78.20%   78.24%   +0.04%     
==========================================
  Files         101      103       +2     
  Lines       14223    14214       -9     
==========================================
- Hits        11123    11122       -1     
+ Misses       3100     3092       -8     

Impacted Files	Coverage Δ
crates/matrix-sdk-appservice/src/event_handler.rs	0.00% <ø> (ø)
crates/matrix-sdk-appservice/src/virtual_user.rs	50.00% <50.00%> (ø)
crates/matrix-sdk-appservice/src/registration.rs	52.00% <52.00%> (ø)
crates/matrix-sdk-appservice/tests/tests.rs	96.17% <70.00%> (-0.40%)	⬇️
crates/matrix-sdk-appservice/src/lib.rs	86.46% <94.73%> (+15.10%)	⬆️

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[gnunicorn]

@johannescpk appservice tests still failing.

[johannescpk]

@gnunicorn Thanks for the poke, seems like now only an unrelated check is failing.