Remove unused oob APIs, move code to libvcx_core

[Patrik-Stas]

1. Remove obscure APIs on OOB receiver handler (which can eventually be ?perhaps? also deleted). Deleted methods:

• connection_exists
• nonmediated_connection_exists
• build_connection

2. Move code only used by libvcx_core from aries_vcx to libvcx_core

• State enums -> u32 conversions
• *_find_message_to_handle functions

3. Rename .*Connection.* types to .*MediatedConnection.* to avoid confusion

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@88f6e8e). Click here to learn what that means.
Report is 2 commits behind head on main.
The diff coverage is 0.00%.

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  Hits            ?      12           
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Flag	Coverage Δ
unittests-aries-vcx	0.05% <0.00%> (?)

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

Files	Coverage Δ
aries_vcx/src/handlers/out_of_band/receiver.rs	0.00% <ø> (ø)
aries_vcx/src/handlers/out_of_band/sender.rs	0.00% <ø> (ø)
aries_vcx/src/handlers/util.rs	0.00% <ø> (ø)
...ocols/mediated_connection/inviter/state_machine.rs	0.00% <0.00%> (ø)
...ocols/mediated_connection/invitee/state_machine.rs	0.00% <0.00%> (ø)
aries_vcx/src/handlers/mediated_connection/mod.rs	0.00% <0.00%> (ø)