Allow SecretManage to be extensible

[DaughterOfMars]

Description of change

Updates the SecretManage trait so that it can be extended by users and provided to a Wallet and Account, thereby enabling its usage via rust. This does not add the ability to pass a custom Secret Manager to the client methods that accept one, however, it may enable us to refactor those methods later to allow passing one in.

Links to any relevant issues

• Closes Use Trait in favor of enum for SecretManager #407

Type of change

Choose a type of change, and delete any options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Open Questions

• I tried to make the types default to use the SecretManager enum, but this only seems to affect the impl blocks. So, I created a new type definition for the default and exported it from the top level. Not sure this is the best way.