CSUB-348 Simplify associated signing types

[CAGS295]

Description of proposed changes:
Get rid of some associated types that were used as intermediate types between authority key selection and offchain-worker signing.

This may not be final but, the changes are in the right direction.

Practical tips for PR review & merge:

• All GitHub Actions report PASS
• Newly added code/functions have unit tests
  • Coverage tools report all newly added lines as covered
  • The positive scenario is exercised
  • Negative scenarios are exercised, e.g. assert on all possible errors
  • Assert on events triggered if applicable
  • Assert on changes made to storage if applicable
• Modified behavior/functions - try to make sure above test items are covered
• Integration tests are added if applicable/needed

[github-actions]

For full LLVM coverage report click here!

[codecov-commenter]

Codecov Report

Merging #815 (482410c) into dev (d21ef32) will decrease coverage by 0.01%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##              dev     #815      +/-   ##
==========================================
- Coverage   78.06%   78.05%   -0.02%     
==========================================
  Files          66       66              
  Lines       10601    10609       +8     
==========================================
+ Hits         8276     8281       +5     
- Misses       2325     2328       +3     

Impacted Files	Coverage Δ
pallets/creditcoin/src/mock.rs	100.00% <ø> (ø)
...allets/offchain-task-scheduler/src/mock/runtime.rs	100.00% <ø> (ø)
pallets/offchain-task-scheduler/src/tasks.rs	100.00% <ø> (ø)
runtime/src/lib.rs	23.78% <ø> (+0.14%)	⬆️
pallets/offchain-task-scheduler/src/lib.rs	95.65% <85.71%> (-1.28%)	⬇️
pallets/creditcoin/src/ocw/tasks/collect_coins.rs	98.95% <100.00%> (ø)
pallets/offchain-task-scheduler/src/ocw.rs	100.00% <100.00%> (ø)
pallets/offchain-task-scheduler/src/ocw/nonce.rs	99.37% <100.00%> (ø)
pallets/offchain-task-scheduler/src/ocw/tests.rs	100.00% <100.00%> (ø)
node/src/main.rs	16.66% <0.00%> (ø)
... and 1 more

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[CAGS295]

blocked by #813

[atodorov]

Minor changes requested.

[atodorov]

LGTM