Change bool flags to str option.

[nstarman]

daft/src/daft/_core.py

Lines 570 to 572 in 7e859f2

	observed=False,
	fixed=False,
	alternate=False,

can be more succinctly represented as

kind: Literal["observed", "fixed", "alternate"] | None = None

This makes it impossible to pass more than one option and still easy to check that the option is correct.