Add ReadOnlySpan<byte> overloads for string parameters

[CasualPokePlayer]

Would allow for taking advantage of C#11 UTF-8 string literals, which can be simply pinned and passed over to ImGui directly (without incurring any memory allocations / no UTF-8 conversions at runtime)

[zaafar]

i know, i want to end 2.0 support #457 and then do this.