XlaTensor refactoring, in preparation to variable i/o types.

[mooskagh]

• Move XlaTensor to a separate file
• Remove string-based tensor buffer, only use heap-based XlaMutableTensor for all use cases.
• Remove special Lc0InputTensor, use "generic" XlaMutableTensor for it
• Both adding a sample to a tensor, and resizing the tensor to be accepted by PjRt Executable is done by the same XlaMutableTensor::Reshape

[mooskagh]

Back to draft, want to add some more stuff.

[mooskagh]

Ready for review