chore: Refactor shape_id -> shape_handle for clearer name

[KyleAMathews]

Addresses one of the renames proposed in https://github.com/electric-sql/electric/issues/1771

• [ ] rename shapeId as well

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[KyleAMathews]

Yeah either way it'll be annoying to rebase... I don't care which way we go.

[balegas]

Is anything left in this PR? Let's merge it if not.

Needs to be coordinated with https://github.com/electric-sql/electric/pull/1833

[thruflo]

I want to say this is superseded by #1900. Although I'm not super hot on git foo so just to be clear: I rebased that PR on this one. So I'm not sure do we need to merge this PR if we merge that one, or not? I guess we can just merge #1900?

[icehaunter]

Superseded by #1900