refactor: make strategy start identities a new struct

[pauldelucia]

Issue being fixed or feature implemented

The way of making Strategy start_identities a tuple of u8s was ugly, so we made it a struct StartIdentities. We also added an optional field where the starting balance of each identity can be specified, in addition to the number of identities and keys per identity.

What was done?

See above

How Has This Been Tested?

Strategy tests in Platform TUI

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

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