feat: Expose binary_elementwise_into_string_amortized for plugin authors, recommend apply_into_string_amortized instead of apply_to_buffer

[MarcoGorelli]

This does a couple of things:

• add binary_elementwise_into_string_amortized as a utility for plugin authors (and it may end up being useful internally if the great expressification continues?) - there's a a few plugins that have binary operations which produce strings which could benefit from this
• remove apply_to_buffer and recommend apply_into_string_amortized to plugin authors instead - their perf is about the same, but the latter has the advantage that the input isn't required to be exactly StringChunked