refactor: constrain environment associated type bounds to ease mocking

[evilrobot-01]

The pallet_contracts::Config bound on the associated types within Environment/Ext means that mocking the environment requires taking a dependency on pallet-contracts and requires adding it to the mock runtime. This is overkill for unit tests.

This PR simply constrains the type bounds to the only type currently required - AccountId. This can obviously be refactored in the future as more of the Ext implementation is used.

Also adds a test for the Prepender, which was the original cause of this refactor requirement.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.35714% with 11 lines in your changes missing coverage. Please review.

Project coverage is 46.23%. Comparing base (f0a0a51) to head (7f447f0).

Files with missing lines	Patch %	Lines
pallets/api/src/extension.rs	64.70%	6 Missing ⚠️
extension/src/decoding.rs	76.47%	0 Missing and 4 partials ⚠️
extension/src/mock.rs	75.00%	1 Missing ⚠️

@@                 Coverage Diff                 @@
##           frank/extension     #265      +/-   ##
===================================================
+ Coverage            45.50%   46.23%   +0.72%     
===================================================
  Files                   40       40              
  Lines                 3685     3707      +22     
  Branches              3685     3707      +22     
===================================================
+ Hits                  1677     1714      +37     
+ Misses                1954     1938      -16     
- Partials                54       55       +1     

Files with missing lines	Coverage Δ
extension/src/environment.rs	100.00% <100.00%> (ø)
extension/src/functions.rs	92.50% <100.00%> (+0.05%)	⬆️
extension/src/lib.rs	94.23% <100.00%> (ø)
runtime/devnet/src/config/api/mod.rs	35.29% <ø> (ø)
extension/src/mock.rs	95.12% <75.00%> (ø)
extension/src/decoding.rs	96.91% <76.47%> (ø)
pallets/api/src/extension.rs	48.33% <64.70%> (+48.33%)	⬆️

[chungquantin]

Brilliant syntax!

[evilrobot-01]

Thats down to Peter! 🙌