Add credential received state transition

[nrempel]

This pull request adds an additional state transition for the credential exchange protocol.

When a holder receives a credential, instead of storing it immediately, the protocol enters a "received" state. The controller can then operate on the credential if it wants before storing it.

When the controller is ready for the credential to be stored in the wallet, it calls /credential_exchange/{id}/store which will carry it along to the next and (currently) state. This state is the same as it was previously.

The agent can be started with the flag --auto-store-credential to automate this step. With this flag passed, the agent behaviour is the same as before.

closes #95

[codecov-io]

Codecov Report

Merging #97 into master will decrease coverage by 0.01%.
The diff coverage is 62.16%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
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- Coverage   72.74%   72.73%   -0.02%     
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  Files         178      178              
  Lines        7471     7503      +32     
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+ Hits         5435     5457      +22     
- Misses       2036     2046      +10