Introduce external account provider into pallet_evm

[dmitrylavrenov]

To simplify #60.

The frontier tests have been passed successfully.

[MOZGIII]

The code structure looks fine!

A few notes:

• We might want to push the trait declaration to a separate crate to keep cleaner abstraction isolation.
• The trait documentation should sufficiently explain what the trait is intended for, and each method's docs explain what it does, when it can be called and why, etc. This is tricky because we can't be too specific as to not refer to a particular implementation, which can possibly update out of sync from the documentation we currently discuss. So we need to be quite abstract in regards to mentioning the code parts (i.e. function names and etc) and explain things in terms of general logic instead.

Let's discuss if in doubt.
Otherwise, this very looks good!

Oh, and one more thing: we need to adjust the rest of the crates too: precompiles, etc... Or is it just all we need?

[dmitrylavrenov]

The code structure looks fine!

A few notes:

• We might want to push the trait declaration to a separate crate to keep cleaner abstraction isolation.
• The trait documentation should sufficiently explain what the trait is intended for, and each method's docs explain what it does, when it can be called and why, etc. This is tricky because we can't be too specific as to not refer to a particular implementation, which can possibly update out of sync from the documentation we currently discuss. So we need to be quite abstract in regards to mentioning the code parts (i.e. function names and etc) and explain things in terms of general logic instead.

Let's discuss if in doubt. Otherwise, this very looks good!

Oh, and one more thing: we need to adjust the rest of the crates too: precompiles, etc... Or is it just all we need?

The trait related docs have been improved. Your suggestion would be great as well!
Moved to separate mod as I was thinking that it's close part of the pallet_evm itself as runner logic for example. Would be happy to understand if it should be declared in separate crate.