Public API needs improvements to make it more extensible

[jessebraham]

There are various public functions in the API, such as load_elf_to_flash, which are not extensible in their current form. These functions should be marked as deprecated and be replaced by functions which are extensible. For instance, by taking a #[non_exhaustive] struct containing the required options.

Since we can deprecate the old functions this can be done in a semver-compatible way. We should identify additional functions which need improving and construct a task list below when able.

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• load_elf_to_flash_with_format
• load_elf_to_flash

[SergioGasquez]

Since we are working towards 3.0 we don't need to deprecate functions, we can just remove them to simplify the codebase.