Add GetCaller for use in error helpers

[prashantv]

Fixes #70

errtrace.Wrap captures the immediate caller, which doesn't compose
with existing error-wrapping helpers. To support this use-case, add
errtrace.GetCaller() that helpers can use to capture their caller
and add that to the error trace.

This approach was chosen over the typical skip callers as:

• Skip requires a more complex assembly implementation to skip an
  arbitrary number of frames, which is more error-prone, e.g.,
  handling the skip argument being too large.

• Faster as each call only skips a fixed (and predictable) number of
  frames. This is especially important since it's expected that
  errtrace is used to annotate all frames in the return path.

• Avoids skip calculations, and generally more flexible to pass around
  compared to skipping frames, which is tied to the stack.

Regardless of which approach is chosen, there is a limitation:
incompatibility with inlined callers. Go tracks inlined code
and PCs of inlined code is correctly mapped back, but the stack
does not contain inlined frames, so the skip calculation done
cannot correctly skip inlined frames. This approach only requires
the error helper to have a stack frame, which is typically the case
when calling errtrace.GetCaller followed by Wrap, but callers
should use go:noinline for correct stack frames.

[prashantv]

This talk on mid-stack inliningwas helpful in understanding how Go tracks inlined PCs:
https://docs.google.com/presentation/d/1Wcblp3jpfeKwA0Y4FOmj63PW52M_qmNqlQkNaLj0P5o/edit#slide=id.p

runtime.Callers skips correctly even in the presence of inline frames using the inlining tables.

(and for reference, how the trace unwinder unwinds)

[prashantv]

This change is part of the following stack:

• Fix lint errors #109
  • Bump go versions to 1.23, 1.22, and 1.21 #110
    • Add GetCaller for use in error helpers #108 ◀

_{Change managed by git-spice.}

[abhinav]

lgtm besides comments