Remove the usage of challenge code

[Kailai-Wang]

resolves #1739

This is a big PR trying to:

• remove IMP mock - it's not used anymore
• replace the challenge code with sidechain nonce and encrypted identities
• deprecate create_identity and verify_identity by merging them into one extrinsic: link_identity
• remove metadata when linking identities - it's never been used so far
• simplify IdentityContext. Now it has two fields only: linkBlock and status
• simplify mock-server getter
• adjust ts-test
• remove some unnecessary test cases

TODO:

• re-benchmark the IMP pallet
• avoid using hardcoded sidechain-nonce in ts-test
• add example code for DI
• other TODOs in ts-test

I'd like to delay the merging of this PR until the other two issues are solved, so that we can have a release with those fixes for IDHub without adding this new link_identity feature:

• Accept wrapped message when verifying the signature of trusted calls  #1752
  DI port on the instance not reachable in browser #1725 it's not a blocker right now (but needs to be fixed asap)

[Kailai-Wang]

I still need to resolve the conflicts, but basically it can be reviewed

edit: conflicts resolved.

[codecov-commenter]

Codecov Report

Merging #1761 (dd946b4) into dev (1f842a2) will decrease coverage by 0.69%.
The diff coverage is 2.77%.

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@@            Coverage Diff             @@
##              dev    #1761      +/-   ##
==========================================
- Coverage   18.65%   17.97%   -0.69%     
==========================================
  Files         273      268       -5     
  Lines       13102    12670     -432     
==========================================
- Hits         2444     2277     -167     
+ Misses      10658    10393     -265     

Impacted Files	Coverage Δ
primitives/core/src/error.rs	0.00% <ø> (ø)
tee-worker/app-libs/stf/src/helpers.rs	0.00% <0.00%> (ø)
tee-worker/app-libs/stf/src/trusted_call.rs	0.00% <0.00%> (ø)
...orker/core-primitives/node-api/metadata/src/lib.rs	0.00% <ø> (ø)
tee-worker/core-primitives/types/src/lib.rs	0.00% <ø> (ø)
...arentchain/indirect-calls-executor/src/executor.rs	0.00% <0.00%> (ø)
...tchain/indirect-calls-executor/src/filter_calls.rs	0.00% <0.00%> (ø)
...-executor/src/indirect_calls/litentry/batch_all.rs	0.00% <0.00%> (ø)
...cutor/src/indirect_calls/litentry/link_identity.rs	0.00% <0.00%> (ø)
...worker/litentry/core/data-providers/src/graphql.rs	0.00% <ø> (ø)
... and 8 more

... and 2 files with indirect coverage changes